More connection management cleanup.

[aahoughton]

Two things, which maybe ought to be split apart:

1. node-redis explicitly clears the event observers on the underlying stream when calling client.end(); this means we'll never get an end event, and we'll never clean our clients array unless we get a sentinel failover. Work around this by explicitly filtering closing clients when creating a new client.
2. I'm vaguely concerned about life cycle for clients that hit a handled error, for whatever reason -- it's not clear to me that we get any closing event (or even that we can reconnect). Create a concept of unmanaged connections, and slide it into the createClient options.

I'm happy to break this into different commits / remove the second, honestly. Thank you!