Added Monitor class to Connection.

[petevg]

Calling Monitor.status allows a client to determine whether the
connection is "connected", "disconnected" or in an "error" state, and
take appropriate action as a result.

This is meant to address the issue of watchers failing silently when the
websocket connection goes away due to a network issue.

@tvansteenburgh @johnsca @abentley Addresses #99. WIP because I want to write some matrix code that uses this and validate that it actually solves my problem. Edit: no longer WIP, as this works quite nicely in matrix :-)

[johnsca]

LGTM.

Do you know if the AllWatcher task goes to done when the connection drops? If not, it would be good to tie this in to that so that the task doesn't sit there waiting for a response that will never come. Actually, an Event interface for detecting connection drop similar to model._watch_received (but public) might be nice.

[petevg]

@johnsca if you drill down the AllWatcher task ends up repeatedly calling connection.recv (via Type.rpc, via AllModelWatcherFacade.Next), which, thanks to a change you made, raises an Exception if the connection unexpectedly closes.

So the future should make it into the "done" state, with an .exception on it. I'm going to go ahead and merge this :-)