Add sentinel support. Implementation possibly could use a few more checks around the restart but it's simple and small

I think the only outstanding task is around the reboot states - especially '-slave-restart-as-master'
In sentinel, if you take down a master when it's in the slave state and restart it, sentinel will drop the current master and make the restarted server the master without triggering a failover.
This should probably trigger a restart of the actor too.
It's not really documented but you can see the message in the source code here:
https://github.com/antirez/redis/blob/unstable/src/sentinel.c#L1550

Otherwise, this should be a correct and complete implementation of a sentinel client.