Cluster: better logging for clusterUpdateSlotsConfigWith().
Cluster: clusterHandleConfigEpochCollision() fixed.

New config epochs must always be obtained incrementing the currentEpoch,
that is itself guaranteed to be >= the max configEpoch currently known
to the node.
Cluster: handshake "already known" error logged to VERBOSE.

This is not really an error but something that always happens for
example when creating a new cluster, or if the sysadmin rejoins manually
a node that is already known.

Since useless logs don't help, moved to VERBOSE level.