Sentinel: always send CONFIG REWRITE when changing instance role.

This change makes Sentinel less fragile about a number of failure modes.

This commit also fixes a different bug as a side effect, SLAVEOF command
was sent multiple times without incrementing the pending commands count.
Sentinel: increment pending_commands counter in two more places.

AUTH and SCRIPT KILL were sent without incrementing the pending commands
counter. Clearly this needs some kind of wrapper doing it for the caller
in order to be less bug prone.
Sentinel: sentinelSendSlaveOf() was missing a var and the prototype.