db_bench: set thread pool size according to max_background_flushes
db_bench: set thread pool size according to max_background_flushes

Summary: option max_background_flushes doesn't make sense if thread pool size is not set accordingly. Set the thread pool size as what we do for max_background_compactions.

Test Plan: Run db_bench with max_background_flushes > 1

Reviewers: yhchiang, igor, rven, ljin

Reviewed By: ljin

Subscribers: MarkCallaghan, leveldb

Differential Revision: https://reviews.facebook.net/D24717

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ksdb,Andymic/rocksdb,OverlordQ/rocksdb,zhangpng/rocksdb,JohnPJenkins/rocksdb,IMCG/RcoksDB,wskplho/rocksdb,mbarbon/rocksdb,ylong/rocksdb,Vaisman/rocksdb,temicai/rocksdb,ryneli/rocksdb,zhangpng/rocksdb,mbarbon/rocksdb,virtdb/rocksdb,jalexanderqed/rocksdb,amyvmiwei/rocksdb,temicai/rocksdb,norton/rocksdb,Applied-Duality/rocksdb,jalexanderqed/rocksdb,Applied-Duality/rocksdb,JohnPJenkins/rocksdb,ryneli/rocksdb,kaschaeffer/rocksdb,caijieming-baidu/rocksdb,virtdb/rocksdb,vashstorm/rocksdb,vmx/rocksdb,mbarbon/rocksdb,wat-ze-hex/rocksdb,anagav/rocksdb,wenduo/rocksdb,ylong/rocksdb,NickCis/rocksdb,tsheasha/rocksdb,zhangpng/rocksdb,wenduo/rocksdb,luckywhu/rocksdb,wskplho/rocksdb,flabby/rocksdb,bbiao/rocksdb,Applied-Duality/rocksdb,alihalabyah/rocksdb,dkorolev/rocksdb,geraldoandradee/rocksdb,hobinyoon/rocksdb,IMCG/RcoksDB,Andymic/rocksdb,Vaisman/rocksdb,IMCG/RcoksDB,norton/rocksdb,RyanTech/rocksdb,flabby/rocksdb,amyvmiwei/rocksdb,JoeWoo/rocksdb,wat-ze-hex/rocksdb,makelivedotnet/rocksdb,Andymic/rocksdb,RyanTech/rocksdb,OverlordQ/rocksdb,caijieming-baidu/rocksdb