Add support for Redis Sentinel

[mikn]

With PubSub support this should be relatively easy to add as Redis Sentinel is mostly a group of servers that publishes the current quorum master over a topic.

Documentation: https://redis.io/topics/sentinel

In a nutshell, the Sentinel implementation should subscribe to all sentinels provided in the connection string, subscribe to the relevant topics, listen for updates and update the "real master" in the connection. The easiest implementation is probably to provide some kind of wrapper for the Pool object that returns connections that are according to the sentinels correct at the time. We may also need to wrap the connection object itself to provide for checks and proper failover behaviour.