Backport improvements for redis-cluster from master (v0.9).

List of changes:

  - The cluster connection sends an ASKING command before retrying
    a command on a different node when Redis returns a -ASK response.
  - The cluster connection can be initialized with a partial list of
    nodes, the full slots map will be fetched from Redis itself using
    the CLUSTER NODES command.
  - The slots map can be optionally retrieved from Redis if the server
    returns a -MOVE response, otherwise only the interested slot will
    be permanently reassigned to the new target node.
  - It is possible to specify a set of common parameters applied to
    connections created on the fly when not part of the current pool
    upon -MOVE and -ASK responses returned by Redis for redirections.
  - $cluster->connect() connects to a random connection in the pool
    instead of forcing the connect operation on all the connections.