Memoizing sentinel client prevents leaking conns

Before, a new connection would be established to the sentinel each time
you tried to discover master and then it leaked. If you are doing that a
lot, it results in an error Errno::EMFILE too many open files.

This code lazily caches the sentinel connections as they are needed.
Merge pull request #3 from dplummer/cache-sentinel-connections

Memoizing sentinel client prevents leaking conns