Trace connections on exception

[jrs65]

This adds a config option for toggling the tracing of connections when they encounter an exception.

[jrs65]

There is an annoying issue that aioredis has now moved to a v2 with a fairly different API. @aelanman ported comet to it (though we haven't deployed it at CHIME), but my attempts to port coco to use it haven't been panning out. I do what I think should work, but it hangs in a redis call and I can't figure out why. So in the meantime we'll need to keep using the old version, and be very careful to not accidentally upgrade it somewhere.

(this incompatibility is also why the tests aren't running at the moment)

[jrs65]

Deployment steps (notes for @jrs65):

• [ ] Change coco.yaml play to
  • Set the branch to this one
  • Remove the coco_server role
• [ ] Run ansible play
• [ ] Manually restart the coco service on csbfs
• [ ] ?
• [ ] Profit