Cluster: clusterSetStartupEpoch() made more generally useful.

The actual goal of the function was to get the max configEpoch found in
the cluster, so make it general by removing the assignment of the max
epoch to currentEpoch that is useful only at startup.
Cluster: configEpoch assignment in SETNODE improved.

Avoid to trash a configEpoch for every slot migrated if this node has
already the max configEpoch across the cluster.

Still work to do in this area but this avoids both ending with a very
high configEpoch without any reason and to flood the system with fsyncs.
Cluster: redis-trib fix: handling of another trivial case.
Cluster: UPDATE messages are the norm and verbose.

Logging them at WARNING level was of little utility and of sure disturb.