Cluster: clusterBroadcastPong() improved with target selection.

Now we can broadcast a pong to all the instances or just the local
slaves (that is useful for replication offset propagation).
Cluster: added a global myself pointer in cluster.c.

Accessing to the 'myself' node, the node representing the currently
running instance, is handy without the need to type
server.cluster->myself every time.
Cluster: use myself instead of server->cluster.myself.
Cluster: refactoring: new macros to check node flags.