Cluster: clusterAcceptHandler() comments updated to match the code.
Cluster: bulk-accept new nodes connections.

The same change was operated for normal client connections. This is
important for Cluster as well, since when a node rejoins the cluster,
when a partition heals or after a restart, it gets flooded with new
connection attempts by all the other nodes trying to form a full
mesh again.
DEBUG POPULATE: call dictExpand() to avoid useless rehashing.