Cluster: added a global myself pointer in cluster.c.
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.

JackieXie168/redis,JackieXie168/redis,JackieXie168/redis,JackieXie168/redis