Add support for listening of connection state changes

[seigert-xx]

In our project we use password-protected Redis instance for writing.

Right after RedisClient creation it is more or less trivial to AUTH to it (and SELECT if it is needed), but if connection was lost afterwards, Rediscala do not provide any useful info about reconnection and also do not store AUTH data in connector actor to authentificate itself automatically.

Thus if we want to re-authenticate ourselves we need to either check Future's failures for "ERR Operation not permitted" messages or parse logs.

I think it would be very useful to provide some form of callback or listener to connection state changes. Also, it could be useful (but not too secure) to store last used AUTH and SELECT data to auto exec them right after reconnection.