Sentinel test: Makefile target added.
Fix configEpoch assignment when a cluster slot gets "closed".

This is still code to rework in order to use agreement to obtain a new
configEpoch when a slot is migrated, however this commit handles the
special case that happens when the nodes are just started and everybody
has a configEpoch of 0. In this special condition to have the maximum
configEpoch is not enough as the special epoch 0 is not unique (all the
others are).

This does not fixes the intrinsic race condition of a failover happening
while we are resharding, that will be addressed later.
Merge branch 'unstable' of github.com:/antirez/redis into unstable
Document why we update peak memory in INFO.