Set server.repl_down_since to 0 when changing master.

When an instance is potentially set to replicate with another master, it
is conceptually disconnected forever, since we have no old copy of the
dataset for this master in memory.
Cluster: allow CLUSTER REPLICATE to switch master.

The code was doing checks for slaves that should be done only when the
instance is currently a master. Switching a slave from a master to
another one should just work.