Track connection statistics on the client

[thatbudakguy]

We have work elsewhere that hijacks the client as a state store to track messages sent and received – it'd be simpler if the client just did that on its own.

If this functionality isn't desired, it could always be in a subclass that wraps the methods with counters, so you can ignore it entirely and use the base class instead.

Outage tracking (how many times the down() flag was set) is useful information too.