Support multiple Redis

[BobDickinson]

Investigating support multiple Redis instances (master-replica).

https://github.com/Faithlife/redis-autopilot

[BobDickinson]

To be clear, we are talking about supporting failover in a master-slave configuration. We are not talking about Redis clustering (sharding).

After further review, we need to use a Redis library with Sentinel support (as in the Faithlife example). They use: https://www.npmjs.com/package/ioredis

Our current library has no Sentinel support: https://www.npmjs.com/package/redis

Sentinel support allows the Redis client to automatically detect and start talking to a new master when there is a Redis failover event resulting in a new master. I believe that this is all automatic from the perspective of the client (that we don't need to be doing ContainerPilot restarts in response to Redis scaling events, for example).

One possible wrinkle is that if you start with a single Redis instance (Redis + Sentinel), we need to make sure that as new instances are added, that the client will pick up that config (it seems like as long as it has any Sentinel configured, it can find others that show up, but there is a warning about making sure to give it at least a couple of Sentinels).

We also need to make sure that the Redis instance(s) is/are registered with Consul before we start Synchro and query Consul for them.