comparator_db_test to cover more irregular comparators
comparator_db_test to cover more irregular comparators

Summary:
comparator_db_test now adds verification for three more comparators:
(1) one that store double as string
(2) one that cast uint64 to string
(3) one that concatenate two strings, prefixing their sizes.
(4) one that order by hash of the string

Test Plan:
Run ./comparator_db_test

Reviewers: ljin, yhchiang, rven, igor

Reviewed By: igor

Subscribers: leveldb, dhruba

Differential Revision: https://reviews.facebook.net/D27927

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