enable the ability to set key size in db_bench in rocksdb
enable the ability to set key size in db_bench in rocksdb

Summary:
1. the default value for key size is still 16
2. enable the ability to set the key size via command line --key_size=

Test Plan:
build & run db_banch and pass some value via command line.
verify it works correctly.

Reviewers: sheki

Reviewed By: sheki

CC: leveldb

Differential Revision: https://reviews.facebook.net/D8943

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