replace boost::iequals with proxygen::caseInsensitiveEqual
replace boost::iequals with proxygen::caseInsensitiveEqual

Summary:
replace boost::iequals with proxygen::caseInsensitiveEqual, which will
save both in binary size and efficiency.

build size on fbcode is reduced from 3281368 to 3246944, saved 3.5KB.

efficiency:
============================================================================
experimental/benyluo/caseInsensitiveEqual/CaseInsensitiveEqualBenchmark.cpprelative  time/iter  iters/s
============================================================================
proxygen_equal                                             841.95ns  1.19M
boost_iequals                                                3.20us  312.69K
============================================================================

Test Plan: unit tests

Reviewed By: seanc@fb.com

Subscribers: trunkagent, bmatheny, seanc

FB internal diff: D1888847

Tasks: 6011097

Signature: t1:1888847:1425508187:1a0cee9e38048fb18af744afd23d463f7faa107f

hiproz/proxygen,sssemil/proxygen,zhenglaizhang/proxygen,luxuan/proxygen,jgli/proxygen,pueril/proxygen,hongliangzhao/proxygen,jgli/proxygen,zhenglaizhang/proxygen,hongliangzhao/proxygen,supriyantomaftuh/proxygen,KublaikhanGeek/proxygen,zhiweicai/proxygen,wrmsr/proxygen,chenmoshushi/proxygen,zhiweicai/proxygen,zhenglaizhang/proxygen,Orvid/proxygen,hiproz/proxygen,supriyantomaftuh/proxygen,fqihangf/proxygen,chjp2046/proxygen,raphaelamorim/proxygen,chenmoshushi/proxygen,jgli/proxygen,luxuan/proxygen,songfj/proxygen,raphaelamorim/proxygen,jgli/proxygen,supriyantomaftuh/proxygen,Orvid/proxygen,LilMeyer/proxygen,LilMeyer/proxygen,Orvid/proxygen,chjp2046/proxygen,KublaikhanGeek/proxygen,zhiweicai/proxygen,LilMeyer/proxygen,Werror/proxygen,raphaelamorim/proxygen,chjp2046/proxygen,chenmoshushi/proxygen,hongliangzhao/proxygen,hnutank163/proxygen,Orvid/proxygen,Werror/proxygen,fqihangf/proxygen,KublaikhanGeek/proxygen,hiproz/proxygen,songfj/proxygen,hnutank163/proxygen,hnutank163/proxygen,wrmsr/proxygen,wrmsr/proxygen,hongliangzhao/proxygen,luxuan/proxygen,Werror/proxygen,sssemil/proxygen,sssemil/proxygen,pueril/proxygen,fqihangf/proxygen,songfj/proxygen,zhiweicai/proxygen,LilMeyer/proxygen,zhenglaizhang/proxygen,raphaelamorim/proxygen,wrmsr/proxygen,supriyantomaftuh/proxygen,KublaikhanGeek/proxygen,sssemil/proxygen,songfj/proxygen,chenmoshushi/proxygen,hnutank163/proxygen,hiproz/proxygen,fqihangf/proxygen,Werror/proxygen,pueril/proxygen,pueril/proxygen,chjp2046/proxygen