Fix a performance regression in thread-pool i/o backend.
Fix a performance regression in thread-pool i/o backend.

4talesa/rethinkdb,tempbottle/rethinkdb,Wilbeibi/rethinkdb,gavioto/rethinkdb,rrampage/rethinkdb,urandu/rethinkdb,bpradipt/rethinkdb,gavioto/rethinkdb,captainpete/rethinkdb,elkingtonmcb/rethinkdb,wujf/rethinkdb,ajose01/rethinkdb,AtnNn/rethinkdb,JackieXie168/rethinkdb,robertjpayne/rethinkdb,4talesa/rethinkdb,victorbriz/rethinkdb,yakovenkodenis/rethinkdb,robertjpayne/rethinkdb,lenstr/rethinkdb,yaolinz/rethinkdb,mquandalle/rethinkdb,wojons/rethinkdb,mquandalle/rethinkdb,tempbottle/rethinkdb,mcanthony/rethinkdb,elkingtonmcb/rethinkdb,RubenKelevra/rethinkdb,eliangidoni/rethinkdb,4talesa/rethinkdb,gavioto/rethinkdb,yakovenkodenis/rethinkdb,nviennot/rethinkdb,eliangidoni/rethinkdb,jmptrader/rethinkdb,elkingtonmcb/rethinkdb,grandquista/rethinkdb,AntouanK/rethinkdb,dparnell/rethinkdb,wkennington/rethinkdb,ayumilong/rethinkdb,Wilbeibi/rethinkdb,niieani/rethinkdb,ajose01/rethinkdb,mbroadst/rethinkdb,Wilbeibi/rethinkdb,wujf/rethinkdb,jmptrader/rethinkdb,eliangidoni/rethinkdb,nviennot/rethinkdb,wkennington/rethinkdb,catroot/rethinkdb,AntouanK/rethinkdb,gavioto/rethinkdb,losywee/rethinkdb,tempbottle/rethinkdb,Wilbeibi/rethinkdb,grandquista/rethinkdb,Wilbeibi/rethinkdb,tempbottle/rethinkdb,losywee/rethinkdb,wojons/rethinkdb,dparnell/rethinkdb,matthaywardwebdesign/rethinkdb,tempbottle/rethinkdb,Wilbeibi/rethinkdb,elkingtonmcb/rethinkdb,mbroadst/rethinkdb,scripni/rethinkdb,sontek/rethinkdb,elkingtonmcb/rethinkdb,matthaywardwebdesign/rethinkdb,jfriedly/rethinkdb,KSanthanam/rethinkdb,mbroadst/rethinkdb,gavioto/rethinkdb,wkennington/rethinkdb,Wilbeibi/rethinkdb,robertjpayne/rethinkdb,gdi2290/rethinkdb,spblightadv/rethinkdb,Qinusty/rethinkdb,jfriedly/rethinkdb,Qinusty/rethinkdb,ajose01/rethinkdb,alash3al/rethinkdb,gdi2290/rethinkdb,captainpete/rethinkdb,bchavez/rethinkdb,victorbriz/rethinkdb,wojons/rethinkdb,bpradipt/rethinkdb,wujf/rethinkdb,bchavez/rethinkdb,bchavez/rethinkdb,greyhwndz/rethinkdb,pap/rethinkdb,ajose01/rethinkdb,grandquista/rethinkdb,tempbottle/rethinkdb,gavioto/rethinkdb,jesseditson/rethinkdb,robertjpayne/rethinkdb,scripni/rethinkdb,urandu/rethinkdb,jfriedly/rethinkdb,pap/rethinkdb,jesseditson/rethinkdb,robertjpayne/rethinkdb,KSanthanam/rethinkdb,urandu/rethinkdb,wojons/rethinkdb,mbroadst/rethinkdb,JackieXie168/rethinkdb,victorbriz/rethinkdb,mcanthony/rethinkdb,greyhwndz/rethinkdb,bchavez/rethinkdb,catroot/rethinkdb,lenstr/rethinkdb,jfriedly/rethinkdb,rrampage/rethinkdb,dparnell/rethinkdb,losywee/rethinkdb,sebadiaz/rethinkdb,AntouanK/rethinkdb,jfriedly/rethinkdb,niieani/rethinkdb,ajose01/rethinkdb,rrampage/rethinkdb,wkennington/rethinkdb,sebadiaz/rethinkdb,gdi2290/rethinkdb,spblightadv/rethinkdb,RubenKelevra/rethinkdb,robertjpayne/rethinkdb,victorbriz/rethinkdb,wkennington/rethinkdb,4talesa/rethinkdb,urandu/rethinkdb,sbusso/rethinkdb,Qinusty/rethinkdb,mquandalle/rethinkdb,wujf/rethinkdb,gdi2290/rethinkdb,matthaywardwebdesign/rethinkdb,yakovenkodenis/rethinkdb,losywee/rethinkdb,robertjpayne/rethinkdb,dparnell/rethinkdb,Qinusty/rethinkdb,lenstr/rethinkdb,yaolinz/rethinkdb,marshall007/rethinkdb,mcanthony/rethinkdb,captainpete/rethinkdb,sontek/rethinkdb,nviennot/rethinkdb,alash3al/rethinkdb,RubenKelevra/rethinkdb,scripni/rethinkdb,ajose01/rethinkdb,lenstr/rethinkdb,mquandalle/rethinkdb,Qinusty/rethinkdb,catroot/rethinkdb,lenstr/rethinkdb,alash3al/rethinkdb,catroot/rethinkdb,bpradipt/rethinkdb,jesseditson/rethinkdb,jmptrader/rethinkdb,sbusso/rethinkdb,sontek/rethinkdb,AtnNn/rethinkdb,KSanthanam/rethinkdb,rrampage/rethinkdb,AtnNn/rethinkdb,ayumilong/rethinkdb,robertjpayne/rethinkdb,victorbriz/rethinkdb,gdi2290/rethinkdb,bpradipt/rethinkdb,dparnell/rethinkdb,mquandalle/rethinkdb,marshall007/rethinkdb,mcanthony/rethinkdb,sbusso/rethinkdb,marshall007/rethinkdb,sbusso/rethinkdb,rrampage/rethinkdb,catroot/rethinkdb,eliangidoni/rethinkdb,ayumilong/rethinkdb,yakovenkodenis/rethinkdb,spblightadv/rethinkdb,ajose01/rethinkdb,eliangidoni/rethinkdb,mcanthony/rethinkdb,tempbottle/rethinkdb,ayumilong/rethinkdb,yakovenkodenis/rethinkdb,sebadiaz/rethinkdb,KSanthanam/rethinkdb,4talesa/rethinkdb,robertjpayne/rethinkdb,ajose01/rethinkdb,rrampage/rethinkdb,elkingtonmcb/rethinkdb,greyhwndz/rethinkdb,jesseditson/rethinkdb,JackieXie168/rethinkdb,bpradipt/rethinkdb,alash3al/rethinkdb,mcanthony/rethinkdb,gavioto/rethinkdb,Qinusty/rethinkdb,rrampage/rethinkdb,mbroadst/rethinkdb,scripni/rethinkdb,AntouanK/rethinkdb,pap/rethinkdb,greyhwndz/rethinkdb,JackieXie168/rethinkdb,elkingtonmcb/rethinkdb,gdi2290/rethinkdb,captainpete/rethinkdb,eliangidoni/rethinkdb,mbroadst/rethinkdb,wkennington/rethinkdb,sebadiaz/rethinkdb,mquandalle/rethinkdb,AtnNn/rethinkdb,niieani/rethinkdb,marshall007/rethinkdb,urandu/rethinkdb,tempbottle/rethinkdb,AntouanK/rethinkdb,ayumilong/rethinkdb,bpradipt/rethinkdb,ayumilong/rethinkdb,wojons/rethinkdb,yaolinz/rethinkdb,sbusso/rethinkdb,wojons/rethinkdb,lenstr/rethinkdb,catroot/rethinkdb,AntouanK/rethinkdb,jfriedly/rethinkdb,nviennot/rethinkdb,RubenKelevra/rethinkdb,RubenKelevra/rethinkdb,wujf/rethinkdb,captainpete/rethinkdb,elkingtonmcb/rethinkdb,niieani/rethinkdb,bpradipt/rethinkdb,scripni/rethinkdb,losywee/rethinkdb,KSanthanam/rethinkdb,matthaywardwebdesign/rethinkdb,nviennot/rethinkdb,sontek/rethinkdb,alash3al/rethinkdb,grandquista/rethinkdb,spblightadv/rethinkdb,sontek/rethinkdb,lenstr/rethinkdb,pap/rethinkdb,pap/rethinkdb,matthaywardwebdesign/rethinkdb,victorbriz/rethinkdb,scripni/rethinkdb,matthaywardwebdesign/rethinkdb,spblightadv/rethinkdb,victorbriz/rethinkdb,wkennington/rethinkdb,yaolinz/rethinkdb,captainpete/rethinkdb,mbroadst/rethinkdb,AntouanK/rethinkdb,Wilbeibi/rethinkdb,urandu/rethinkdb,sebadiaz/rethinkdb,ayumilong/rethinkdb,yaolinz/rethinkdb,KSanthanam/rethinkdb,JackieXie168/rethinkdb,gdi2290/rethinkdb,captainpete/rethinkdb,greyhwndz/rethinkdb,losywee/rethinkdb,niieani/rethinkdb,AtnNn/rethinkdb,alash3al/rethinkdb,marshall007/rethinkdb,mbroadst/rethinkdb,4talesa/rethinkdb,sontek/rethinkdb,RubenKelevra/rethinkdb,sebadiaz/rethinkdb,sebadiaz/rethinkdb,wujf/rethinkdb,urandu/rethinkdb,losywee/rethinkdb,niieani/rethinkdb,AntouanK/rethinkdb,KSanthanam/rethinkdb,bpradipt/rethinkdb,mbroadst/rethinkdb,catroot/rethinkdb,RubenKelevra/rethinkdb,wkennington/rethinkdb,bchavez/rethinkdb,yaolinz/rethinkdb,sebadiaz/rethinkdb,jesseditson/rethinkdb,matthaywardwebdesign/rethinkdb,gavioto/rethinkdb,sbusso/rethinkdb,niieani/rethinkdb,sbusso/rethinkdb,matthaywardwebdesign/rethinkdb,alash3al/rethinkdb,pap/rethinkdb,jmptrader/rethinkdb,yakovenkodenis/rethinkdb,jmptrader/rethinkdb,jfriedly/rethinkdb,wojons/rethinkdb,grandquista/rethinkdb,mcanthony/rethinkdb,dparnell/rethinkdb,victorbriz/rethinkdb,JackieXie168/rethinkdb,bchavez/rethinkdb,bchavez/rethinkdb,greyhwndz/rethinkdb,RubenKelevra/rethinkdb,pap/rethinkdb,yakovenkodenis/rethinkdb,bpradipt/rethinkdb,jesseditson/rethinkdb,lenstr/rethinkdb,JackieXie168/rethinkdb,AtnNn/rethinkdb,yaolinz/rethinkdb,AtnNn/rethinkdb,dparnell/rethinkdb,mquandalle/rethinkdb,scripni/rethinkdb,urandu/rethinkdb,marshall007/rethinkdb,nviennot/rethinkdb,nviennot/rethinkdb,eliangidoni/rethinkdb,AtnNn/rethinkdb,eliangidoni/rethinkdb,Qinusty/rethinkdb,captainpete/rethinkdb,sbusso/rethinkdb,jesseditson/rethinkdb,Qinusty/rethinkdb,bchavez/rethinkdb,jmptrader/rethinkdb,spblightadv/rethinkdb,pap/rethinkdb,Qinusty/rethinkdb,marshall007/rethinkdb,4talesa/rethinkdb,catroot/rethinkdb,JackieXie168/rethinkdb,bchavez/rethinkdb,alash3al/rethinkdb,greyhwndz/rethinkdb,losywee/rethinkdb,mcanthony/rethinkdb,niieani/rethinkdb,sontek/rethinkdb,marshall007/rethinkdb,nviennot/rethinkdb,KSanthanam/rethinkdb,yakovenkodenis/rethinkdb,4talesa/rethinkdb,ayumilong/rethinkdb,wujf/rethinkdb,scripni/rethinkdb,grandquista/rethinkdb,wojons/rethinkdb,jmptrader/rethinkdb,jesseditson/rethinkdb,jfriedly/rethinkdb,eliangidoni/rethinkdb,spblightadv/rethinkdb,mquandalle/rethinkdb,grandquista/rethinkdb,yaolinz/rethinkdb,JackieXie168/rethinkdb,grandquista/rethinkdb,sontek/rethinkdb,dparnell/rethinkdb,jmptrader/rethinkdb,spblightadv/rethinkdb,grandquista/rethinkdb,dparnell/rethinkdb,greyhwndz/rethinkdb,rrampage/rethinkdb