Initial JedisSentinelPool based on code from hamsterready/jedis-sentinel-pool.
Given a master name and a set of sentinels this pool will subscribe to the
Sentinel notifications about master changes (+switch-master) and
re-establishes the pool to the new master when one is announced.
Refactored the inline thread that monitors the master and turned it into its
own class. The pool shuts down the monitors as well now.