Small speedup of CompactionFilterV2
Small speedup of CompactionFilterV2

Summary: ToString() is expensive. Profiling shows that most compaction threads are stuck in jemalloc, allocating a new string. This will help out a litte.

Test Plan: make check

Reviewers: haobo, danguo

Reviewed By: danguo

CC: leveldb

Differential Revision: https://reviews.facebook.net/D17583

alihalabyah/rocksdb,dkorolev/rocksdb,skunkwerks/rocksdb,fengshao0907/rocksdb,Applied-Duality/rocksdb,facebook/rocksdb,biddyweb/rocksdb,siddhartharay007/rocksdb,JohnPJenkins/rocksdb,vmx/rocksdb,JoeWoo/rocksdb,temicai/rocksdb,vashstorm/rocksdb,rDSN-Projects/rocksdb.replicated,JackLian/rocksdb,norton/rocksdb,tschottdorf/rocksdb,alihalabyah/rocksdb,JackLian/rocksdb,rDSN-Projects/rocksdb.replicated,tschottdorf/rocksdb,JoeWoo/rocksdb,wenduo/rocksdb,RyanTech/rocksdb,sorphi/rocksdb,jalexanderqed/rocksdb,Andymic/rocksdb,temicai/rocksdb,Vaisman/rocksdb,Applied-Duality/rocksdb,lgscofield/rocksdb,biddyweb/rocksdb,rDSN-Projects/rocksdb.replicated,JackLian/rocksdb,kaschaeffer/rocksdb,luckywhu/rocksdb,mbarbon/rocksdb,amyvmiwei/rocksdb,ylong/rocksdb,anagav/rocksdb,JackLian/rocksdb,ylong/rocksdb,wenduo/rocksdb,anagav/rocksdb,wskplho/rocksdb,fengshao0907/rocksdb,OverlordQ/rocksdb,Andymic/rocksdb,mbarbon/rocksdb,Andymic/rocksdb,dkorolev/rocksdb,bbiao/rocksdb,JackLian/rocksdb,wskplho/rocksdb,vashstorm/rocksdb,rDSN-Projects/rocksdb.replicated,Vaisman/rocksdb,dkorolev/rocksdb,wenduo/rocksdb,ryneli/rocksdb,facebook/rocksdb,Applied-Duality/rocksdb,zhangpng/rocksdb,bbiao/rocksdb,tsheasha/rocksdb,anagav/rocksdb,makelivedotnet/rocksdb,zhangpng/rocksdb,JoeWoo/rocksdb,jalexanderqed/rocksdb,JoeWoo/rocksdb,OverlordQ/rocksdb,hobinyoon/rocksdb,siddhartharay007/rocksdb,caijieming-baidu/rocksdb,rDSN-Projects/rocksdb.replicated,mbarbon/rocksdb,wlqGit/rocksdb,temicai/rocksdb,vmx/rocksdb,skunkwerks/rocksdb,makelivedotnet/rocksdb,jalexanderqed/rocksdb,skunkwerks/rocksdb,fengshao0907/rocksdb,sorphi/rocksdb,wskplho/rocksdb,caijieming-baidu/rocksdb,vmx/rocksdb,amyvmiwei/rocksdb,JackLian/rocksdb,Vaisman/rocksdb,fengshao0907/rocksdb,mbarbon/rocksdb,norton/rocksdb,tizzybec/rocksdb,anagav/rocksdb,wlqGit/rocksdb,SunguckLee/RocksDB,mbarbon/rocksdb,NickCis/rocksdb,luckywhu/rocksdb,temicai/rocksdb,norton/rocksdb,makelivedotnet/rocksdb,caijieming-baidu/rocksdb,virtdb/rocksdb,virtdb/rocksdb,IMCG/RcoksDB,NickCis/rocksdb,SunguckLee/RocksDB,flabby/rocksdb,tsheasha/rocksdb,amyvmiwei/rocksdb,Applied-Duality/rocksdb,rDSN-Projects/rocksdb.replicated,RyanTech/rocksdb,kaschaeffer/rocksdb,ryneli/rocksdb,amyvmiwei/rocksdb,JohnPJenkins/rocksdb,norton/rocksdb,alihalabyah/rocksdb,jalexanderqed/rocksdb,facebook/rocksdb,wat-ze-hex/rocksdb,SunguckLee/RocksDB,fengshao0907/rocksdb,geraldoandradee/rocksdb,facebook/rocksdb,biddyweb/rocksdb,makelivedotnet/rocksdb,zhangpng/rocksdb,ryneli/rocksdb,Applied-Duality/rocksdb,tsheasha/rocksdb,wlqGit/rocksdb,virtdb/rocksdb,IMCG/RcoksDB,msb-at-yahoo/rocksdb,lgscofield/rocksdb,dkorolev/rocksdb,dkorolev/rocksdb,flabby/rocksdb,IMCG/RcoksDB,makelivedotnet/rocksdb,sorphi/rocksdb,tsheasha/rocksdb,siddhartharay007/rocksdb,mbarbon/rocksdb,zhangpng/rocksdb,skunkwerks/rocksdb,wat-ze-hex/rocksdb,JohnPJenkins/rocksdb,tizzybec/rocksdb,tizzybec/rocksdb,norton/rocksdb,alihalabyah/rocksdb,geraldoandradee/rocksdb,wlqGit/rocksdb,Andymic/rocksdb,temicai/rocksdb,ylong/rocksdb,bbiao/rocksdb,kaschaeffer/rocksdb,luckywhu/rocksdb,geraldoandradee/rocksdb,biddyweb/rocksdb,tsheasha/rocksdb,hobinyoon/rocksdb,OverlordQ/rocksdb,OverlordQ/rocksdb,kaschaeffer/rocksdb,ryneli/rocksdb,fengshao0907/rocksdb,rDSN-Projects/rocksdb.replicated,tizzybec/rocksdb,SunguckLee/RocksDB,Vaisman/rocksdb,RyanTech/rocksdb,wenduo/rocksdb,wskplho/rocksdb,siddhartharay007/rocksdb,geraldoandradee/rocksdb,wskplho/rocksdb,hobinyoon/rocksdb,lgscofield/rocksdb,jalexanderqed/rocksdb,tizzybec/rocksdb,vashstorm/rocksdb,facebook/rocksdb,bbiao/rocksdb,vmx/rocksdb,caijieming-baidu/rocksdb,JohnPJenkins/rocksdb,Andymic/rocksdb,OverlordQ/rocksdb,RyanTech/rocksdb,tschottdorf/rocksdb,Vaisman/rocksdb,wat-ze-hex/rocksdb,zhangpng/rocksdb,alihalabyah/rocksdb,wlqGit/rocksdb,RyanTech/rocksdb,msb-at-yahoo/rocksdb,temicai/rocksdb,amyvmiwei/rocksdb,wskplho/rocksdb,dkorolev/rocksdb,skunkwerks/rocksdb,skunkwerks/rocksdb,alihalabyah/rocksdb,NickCis/rocksdb,SunguckLee/RocksDB,wenduo/rocksdb,virtdb/rocksdb,zhangpng/rocksdb,vmx/rocksdb,siddhartharay007/rocksdb,ryneli/rocksdb,IMCG/RcoksDB,mbarbon/rocksdb,tsheasha/rocksdb,sorphi/rocksdb,RyanTech/rocksdb,flabby/rocksdb,JohnPJenkins/rocksdb,zhangpng/rocksdb,hobinyoon/rocksdb,wenduo/rocksdb,lgscofield/rocksdb,wat-ze-hex/rocksdb,hobinyoon/rocksdb,RyanTech/rocksdb,siddhartharay007/rocksdb,tizzybec/rocksdb,Andymic/rocksdb,tizzybec/rocksdb,biddyweb/rocksdb,ylong/rocksdb,ryneli/rocksdb,hobinyoon/rocksdb,geraldoandradee/rocksdb,geraldoandradee/rocksdb,kaschaeffer/rocksdb,geraldoandradee/rocksdb,luckywhu/rocksdb,temicai/rocksdb,kaschaeffer/rocksdb,wat-ze-hex/rocksdb,anagav/rocksdb,dkorolev/rocksdb,ylong/rocksdb,fengshao0907/rocksdb,hobinyoon/rocksdb,Vaisman/rocksdb,OverlordQ/rocksdb,Andymic/rocksdb,jalexanderqed/rocksdb,siddhartharay007/rocksdb,wat-ze-hex/rocksdb,amyvmiwei/rocksdb,norton/rocksdb,SunguckLee/RocksDB,JoeWoo/rocksdb,tschottdorf/rocksdb,bbiao/rocksdb,anagav/rocksdb,alihalabyah/rocksdb,IMCG/RcoksDB,bbiao/rocksdb,IMCG/RcoksDB,bbiao/rocksdb,wskplho/rocksdb,luckywhu/rocksdb,sorphi/rocksdb,JoeWoo/rocksdb,norton/rocksdb,anagav/rocksdb,lgscofield/rocksdb,wlqGit/rocksdb,NickCis/rocksdb,flabby/rocksdb,virtdb/rocksdb,vmx/rocksdb,virtdb/rocksdb,SunguckLee/RocksDB,facebook/rocksdb,vashstorm/rocksdb,vashstorm/rocksdb,ylong/rocksdb,tschottdorf/rocksdb,JackLian/rocksdb,msb-at-yahoo/rocksdb,JohnPJenkins/rocksdb,amyvmiwei/rocksdb,vashstorm/rocksdb,JohnPJenkins/rocksdb,wenduo/rocksdb,lgscofield/rocksdb,luckywhu/rocksdb,biddyweb/rocksdb,flabby/rocksdb,msb-at-yahoo/rocksdb,skunkwerks/rocksdb,wlqGit/rocksdb,makelivedotnet/rocksdb,jalexanderqed/rocksdb,Applied-Duality/rocksdb,caijieming-baidu/rocksdb,SunguckLee/RocksDB,NickCis/rocksdb,Andymic/rocksdb,OverlordQ/rocksdb,IMCG/RcoksDB,vmx/rocksdb,wat-ze-hex/rocksdb,caijieming-baidu/rocksdb,facebook/rocksdb,NickCis/rocksdb,lgscofield/rocksdb,sorphi/rocksdb,caijieming-baidu/rocksdb,makelivedotnet/rocksdb,Applied-Duality/rocksdb,flabby/rocksdb,wenduo/rocksdb,wat-ze-hex/rocksdb,kaschaeffer/rocksdb,NickCis/rocksdb,tschottdorf/rocksdb,vashstorm/rocksdb,hobinyoon/rocksdb,bbiao/rocksdb,luckywhu/rocksdb,msb-at-yahoo/rocksdb,facebook/rocksdb,sorphi/rocksdb,norton/rocksdb,Vaisman/rocksdb,msb-at-yahoo/rocksdb,flabby/rocksdb,biddyweb/rocksdb,JoeWoo/rocksdb,virtdb/rocksdb,tsheasha/rocksdb,tsheasha/rocksdb,vmx/rocksdb,ylong/rocksdb,ryneli/rocksdb