support sticky slave whose slaveof config won't be changed by Redis sentinel

This is for mirroring Redis cluster, suppose we have two datacenters,
DC A has 3 Redis instances that are maintained by Redis sentinel,
DC B has 3 Redis instances that synchronize from DC A with fixed
one-one mapping, aka. B_i slaveof A_i.

Without this feature, Redis sentinel will make all slaves synchronize
from single master sooner or later during master election, this will
consume big bandwidth of the master.