fix db_bench to use HashSkipList for real
fix db_bench to use HashSkipList for real

Summary:
For HashSkipList case, DBImpl has sanity check to see if prefix_extractor in
options is the same as the one in memtable factory. If not, it falls
back to SkipList. As result, I was experimenting with SkipList
performance. No wonder it is much worse than LinkedList

Test Plan: ran benchmark

Reviewers: haobo, sdong, igor

Reviewed By: igor

CC: leveldb

Differential Revision: https://reviews.facebook.net/D16569

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