folly/AtomicHashArray: use an unsigned type for each of two counters
folly/AtomicHashArray: use an unsigned type for each of two counters

Summary:
* folly/AtomicHashArray.h (numEntries_, numPendingEntries_):
Use an unsigned type for each of these.  They count things, can
never go below 0, and are compared to unsigned values.
Otherwise, gcc-4.9 would emit this:
folly/AtomicHashArray-inl.h:153:38: error: comparison between signed and unsigned integer expressions [-Werror=sign-compare]

Test Plan:
Run this and note there are fewer errors than before:
fbconfig --platform-all=gcc-4.9-glibc-2.20 -r folly && fbmake dbgo

Reviewed By: philipp@fb.com

Subscribers: trunkagent, folly-diffs@

FB internal diff: D1770695

Tasks: 5941250

Signature: t1:1770695:1420683354:bfa4775bc2f8aab74e34772308a5c8b1779243b8

loverszhaokai/folly,rklabs/folly,tomhughes/folly,arg0/folly,upsoft/folly,Hincoin/folly,Orvid/folly,gavioto/folly,alexst07/folly,KSreeHarsha/folly,facebook/folly,mqeizi/folly,Orvid/folly,facebook/folly,colemancda/folly,bowlofstew/folly,brunomorishita/folly,bowlofstew/folly,theiver9827/folly,facebook/folly,rklabs/folly,arg0/folly,bikong2/folly,SammyK/folly,alexst07/folly,gavioto/folly,chjp2046/folly,loverszhaokai/folly,romange/folly,fw1121/folly,guker/folly,raphaelamorim/folly,project-zerus/folly,PoisonBOx/folly,leolujuyi/folly,PPC64/folly,clearlylin/folly,tempbottle/folly,bowlofstew/folly,stonegithubs/folly,rklabs/folly,mqeizi/folly,doctaweeks/folly,hongliangzhao/folly,constantine001/folly,bsampath/folly,floxard/folly,nickhen/folly,sonnyhu/folly,project-zerus/folly,Eagle-X/folly,leolujuyi/folly,guker/folly,PoisonBOx/folly,tomhughes/folly,renyinew/folly,CJstar/folly,yangjin-unique/folly,raphaelamorim/folly,CJstar/folly,upsoft/folly,clearlinux/folly,cppfool/folly,stonegithubs/folly,romange/folly,constantine001/folly,project-zerus/folly,shaobz/folly,brunomorishita/folly,renyinew/folly,arg0/folly,SeanRBurton/folly,KSreeHarsha/folly,gaoyingie/folly,xzmagic/folly,guker/folly,tempbottle/folly,yangjin-unique/folly,xzmagic/folly,chjp2046/folly,bobegir/folly,floxard/folly,sakishum/folly,Hincoin/folly,SammyK/folly,kernelim/folly,shaobz/folly,sakishum/folly,rklabs/folly,kernelim/folly,Hincoin/folly,gaoyingie/folly,bobegir/folly,ykarlsbrun/folly,sonnyhu/folly,doctaweeks/folly,bowlofstew/folly,charsyam/folly,SammyK/folly,SeanRBurton/folly,zhiweicai/folly,theiver9827/folly,juniway/folly,brunomorishita/folly,wildinto/folly,constantine001/folly,loversInJapan/folly,raphaelamorim/folly,sakishum/folly,bikong2/folly,charsyam/folly,shaobz/folly,renyinew/folly,doctaweeks/folly,wildinto/folly,charsyam/folly,loversInJapan/folly,Eagle-X/folly,SeanRBurton/folly,clearlylin/folly,colemancda/folly,romange/folly,cole14/folly,constantine001/folly,sonnyhu/folly,brunomorishita/folly,leolujuyi/folly,tomhughes/folly,chjp2046/folly,cppfool/folly,PoisonBOx/folly,loverszhaokai/folly,ykarlsbrun/folly,cppfool/folly,mqeizi/folly,facebook/folly,Orvid/folly,arg0/folly,PoisonBOx/folly,lifei/folly,Eagle-X/folly,chjp2046/folly,gaoyingie/folly,lifei/folly,loversInJapan/folly,bobegir/folly,bowlofstew/folly,bsampath/folly,bsampath/folly,yangjin-unique/folly,nickhen/folly,clearlinux/folly,zhiweicai/folly,alexst07/folly,stonegithubs/folly,clearlylin/folly,hongliangzhao/folly,xzmagic/folly,cppfool/folly,tomhughes/folly,stonegithubs/folly,loverszhaokai/folly,fw1121/folly,lifei/folly,reddit/folly,bsampath/folly,zhiweicai/folly,reddit/folly,theiver9827/folly,charsyam/folly,Orvid/folly,floxard/folly,mqeizi/folly,project-zerus/folly,project-zerus/folly,romange/folly,loversInJapan/folly,gavioto/folly,leolujuyi/folly,wildinto/folly,arg0/folly,colemancda/folly,xzmagic/folly,leolujuyi/folly,KSreeHarsha/folly,brunomorishita/folly,floxard/folly,nickhen/folly,facebook/folly,sakishum/folly,bikong2/folly,clearlylin/folly,reddit/folly,chjp2046/folly,clearlinux/folly,nickhen/folly,reddit/folly,hongliangzhao/folly,raphaelamorim/folly,KSreeHarsha/folly,upsoft/folly,Orvid/folly,alexst07/folly,guker/folly,PPC64/folly,gavioto/folly,shaobz/folly,nickhen/folly,CJstar/folly,lifei/folly,raphaelamorim/folly,gaoyingie/folly,kernelim/folly,constantine001/folly,Hincoin/folly,kernelim/folly,hongliangzhao/folly,doctaweeks/folly,romange/folly,juniway/folly,colemancda/folly,bobegir/folly,theiver9827/folly,bobegir/folly,alexst07/folly,renyinew/folly,PPC64/folly,loversInJapan/folly,cole14/folly,floxard/folly,cole14/folly,cole14/folly,SammyK/folly,bikong2/folly,Eagle-X/folly,CJstar/folly,bikong2/folly,fw1121/folly,ykarlsbrun/folly,fw1121/folly,loverszhaokai/folly,juniway/folly,wildinto/folly,SammyK/folly,rklabs/folly,PoisonBOx/folly,wildinto/folly,clearlinux/folly,tempbottle/folly,xzmagic/folly,yangjin-unique/folly,Hincoin/folly,lifei/folly,fw1121/folly,ykarlsbrun/folly,CJstar/folly,PPC64/folly,kernelim/folly,theiver9827/folly,zhiweicai/folly,PPC64/folly,guker/folly,reddit/folly,Eagle-X/folly,zhiweicai/folly,renyinew/folly,cppfool/folly,clearlinux/folly,shaobz/folly,SeanRBurton/folly,mqeizi/folly,tempbottle/folly,juniway/folly,SeanRBurton/folly,KSreeHarsha/folly,tempbottle/folly,upsoft/folly,cole14/folly,sakishum/folly,clearlylin/folly,doctaweeks/folly,hongliangzhao/folly,sonnyhu/folly,bsampath/folly,ykarlsbrun/folly,upsoft/folly,gavioto/folly,gaoyingie/folly,tomhughes/folly,sonnyhu/folly,colemancda/folly,juniway/folly,stonegithubs/folly,yangjin-unique/folly