Track connection staleness

[whereswaldon]

There are circumstances right now in which your client will be disconnected from the server without being aware of it until it tries to actively communicate. It would be a much better user experience if we caught these proactively.

Essentially, I propose that:

• We keep a timer of how long it has been since we received data from the server
• Whenever a message of any kind comes from the server, we reset that timer
• When the timer hits some threshold (30 seconds?), we send an innocuous message that should yield a response from the server (right now, I think this should be a query for the root message).
• If the timer ever hits 60 seconds, we should close and reopen the connection to the server.

I also would like to keep the value within this timer visible to the user somewhere within the TUI.

[whereswaldon]

Everything except making the timer visible was implemented by #53