cluster.c: unblock blocking client when setack
cluster.c: unblock blocking client when setack

When setack msg arrives, we should unblock client who is waiting
for enough replicate.
See issue #73

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