dict.c: dictGetRandomKeys() optimization for big->small table case.
dict.c: dictGetRandomKeys() optimization for big->small table case.

Related to issue #2306.

antirez/disque,lamby/pkg-disque,GrimDerp/disque,GrimDerp/disque,lamby/disque,chen-tian/disque,mopuri-sandeep/disque,lamby/pkg-disque,jmptrader/disque,c2j/disque,ErikDubbelboer/disque,themage/disque,felixbuenemann/disque,CodyReichert/disque,hanicker/disque,gideonairex/disque,ErikDubbelboer/disque,seppo0010/disque,hanicker/disque,douxiaozhao/disque,sunheehnus/disque,Koed00/disque,lamby/disque,CodyReichert/disque,themage/disque,tempbottle/disque,tempbottle/disque,chen-tian/disque,themage/disque,jmptrader/disque,jmptrader/disque,mopuri-sandeep/disque,Koed00/disque,gideonairex/disque,seppo0010/disque,c2j/disque,antirez/disque,sunheehnus/disque,felixbuenemann/disque,douxiaozhao/disque