Cluster: don't gossip about nodes that are not useful to the cluster.
Cluster: new field in cluster node structure, "numslots".

Before a relatively slow popcount() operation was needed every time we
needed to get the number of slots served by a given cluster node.
Now we just need to check an integer that is taken in sync with the
bitmap.
Cluster: use node->numslots instead of popcount() where possible.
Cluster: refactoring of clusterNode*Bit to use helper bitmap functions.
Cluster: better handling of slots changes in PONG packets.

The new code makes sure that the node slots bitmap is always consistent
with the cluster->slots array.
Cluster: don't set the slot as unassigned because of PONG info.

As stated in the comment this is usually due to a resharding in progress
so the client should be still redirected to the old node that will
handle the redirection elsewhere.