Listen on Sentinel PUBSUB channel for +switch-master
Fix the Pub/Sub implementation to fix unit tests and not have a singleton.

Problem:
    The original design of the sentinel with Pub/Sub to the switch-master
    message broke the last unit test because it was always connected to the
    previously successful servers.

Analysis:
    In order to not have always stay connected to the old servers the
    subscription to the Pub/Sub message needs to only occur when the
    successful connect occurs.
Pub/Sub with the sentinel