dict.c: dictGetRandomKeys() visit pattern optimization.
dict.c: dictGetRandomKeys() visit pattern optimization.

We use the invariant that the original table ht[0] is never populated up
to the index before the current rehashing index.

Related to issue #2306.

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