close some idiotic race conditions
close some idiotic race conditions

do_item_update could decide to update an item, then wait on the cache_lock,
but the item could be unlinked in the meantime.

caused this to happen on purpose by flooding with sets, then flushing
repeatedly. flush has to unlink items until it hits the previous second.

niaomingjian/memcached,13693100472/memcached,13693100472/memcached,niaomingjian/memcached,surajdharmapuram/memcached,memcached/memcached,msg7086/repcached,DSrcl/memcached,elfchief/memcached,rmccullagh/memcached,samanbarghi/memcached,alexandermerritt/memcached,13693100472/memcached,rhli/memcached,niaomingjian/memcached,myPublicGit/memcached,leverich/memcached-affinity,bpzhang/memcached,bpzhang/memcached,kamyu104/memcached,allen8807/memcached-1,gencer/memcached,williamcheungthu/memcached,gencer/memcached,leverich/memcached-affinity,memcached/memcached,elfchief/memcached,samanbarghi/memcached,sagark/memcached-accel,mdounin/memcached,elfchief/memcached,angusfenying/memcached-win64,mdounin/memcached,DSrcl/memcached,samt42/memcached,Danzeer/memcached,fengshao0907/memcached,alexandermerritt/memcached,DigitalAce/memcached,neutronsharc/memcached,allen8807/memcached-1,guikcd/memcached,tomykaira/memcached,dianerdianer/memcached,samt42/memcached,amyvmiwei/memcached,Dhruva6/memcached,samanbarghi/memcached,tmharrisonwang/memcached,angusfenying/memcached-win64,ClearwaterCore/memcached,Distrotech/memcached,myPublicGit/memcached,memcached/memcached,williamcheungthu/memcached,surajdharmapuram/memcached,cloudrain21/memcached,myPublicGit/memcached,sagark/memcached-accel,mitliucak/memcached,amyvmiwei/memcached,romanticode/memcached,kamyu104/memcached,xiaosl/memcached,myPublicGit/memcached,bpzhang/memcached,jbergstroem/memcached,rmccullagh/memcached,Danzeer/memcached,ZhangZhenhua/memcached,jeremiahyan/memcached,DSrcl/memcached,elfchief/memcached,emcconville/memcached,gencer/memcached,DigitalAce/memcached,rmccullagh/memcached,gencer/memcached,samt42/memcached,rhli/memcached,leedc/memcached,gencer/memcached,guikcd/memcached,williamcheungthu/memcached,mitliucak/memcached,neutronsharc/memcached,dkrotx/memcached-dd,dormando/memcached,samanbarghi/memcached,cloudrain21/memcached,neutronsharc/memcached,romanticode/memcached,ClearwaterCore/memcached,rmccullagh/memcached,dormando/memcached,fengshao0907/memcached,Dhruva6/memcached,dkrotx/memcached-dd,leverich/memcached-affinity,zaycev/memcached,niaomingjian/memcached,romanticode/memcached,dianerdianer/memcached,tellapart/memcached,ClearwaterCore/memcached,dustin/memcached,DigitalAce/memcached,leedc/memcached,cloudrain21/memcached,zaycev/memcached,Dhruva6/memcached,xiaosl/memcached,dormando/memcached,bpzhang/memcached,fengshao0907/memcached,msg7086/repcached,dkrotx/memcached-dd,tomykaira/memcached,zaycev/memcached,DSrcl/memcached,ZhangZhenhua/memcached,mitliucak/memcached,jeremiahyan/memcached,tellapart/memcached,dormando/memcached,xiaosl/memcached,fengshao0907/memcached,Dhruva6/memcached,dustin/memcached,msg7086/repcached,allen8807/memcached-1,tmharrisonwang/memcached,dormando/memcached,samt42/memcached,tellapart/memcached,memcached/memcached,memcached/memcached,amyvmiwei/memcached,amyvmiwei/memcached,surajdharmapuram/memcached,ZhangZhenhua/memcached,Danzeer/memcached,sagark/memcached-accel,williamcheungthu/memcached,kamyu104/memcached,msg7086/repcached,jeremiahyan/memcached,cloudrain21/memcached,zaycev/memcached,sagark/memcached-accel,EdwardLee03/memcached,emcconville/memcached,dianerdianer/memcached,tomykaira/memcached,romanticode/memcached,alexandermerritt/memcached,guikcd/memcached,jeremiahyan/memcached,dustin/memcached,mitliucak/memcached,angusfenying/memcached-win64,leedc/memcached,guikcd/memcached,neutronsharc/memcached,rmccullagh/memcached,tmharrisonwang/memcached,13693100472/memcached,leedc/memcached,elfchief/memcached,tmharrisonwang/memcached,EdwardLee03/memcached,allen8807/memcached-1,jbergstroem/memcached,ZhangZhenhua/memcached,mdounin/memcached,alexandermerritt/memcached,sagark/memcached-accel,EdwardLee03/memcached,EdwardLee03/memcached,rhli/memcached,dianerdianer/memcached,tomykaira/memcached,13693100472/memcached,rhli/memcached,ClearwaterCore/memcached,jbergstroem/memcached,Distrotech/memcached,kamyu104/memcached,msg7086/repcached,angusfenying/memcached-win64,Distrotech/memcached,emcconville/memcached,dkrotx/memcached-dd,Danzeer/memcached,surajdharmapuram/memcached,guikcd/memcached,emcconville/memcached,xiaosl/memcached