Include Monky command server status in the mode line

[ncalexan]

It's difficult to know when Mercurial is busy after Monky spawns long-running tasks. We should use the mode line to show when hg is churning.

[ncalexan]

While we're talking about command server interaction improvements, it would be helpful to more clearly delineate commands in the *monky-server* buffer (including with status exit codes, which magit does very well); and it would also be helpful to include timings for commands that take more than, say, 5s. I mostly work on a large repository (https://hg.mozilla.org/mozilla-central) and it can be pretty helpful to know what the average times for commands are.