Datadog plugin should handle exception while connection timeout

This PR intends to fix the bug we see in prod:
https://www.pivotaltracker.com/n/projects/956238/stories/80315268

change the healthz to use EM in order to detect EM thread blocked in open connections

Correct me if I'm wrong but it looks like you are trying to block the /healthz request in case the EM thread pool has been exhausted. The problem is that EM.schedule executes on the reactor thread, which should not be blocked in that situation. So, Monit will still get a 200 right away even though the requests to Data Dog are queueing up.

Assuming that was the intent of this change, it seems what you could do here is using EM.defer with some sort of async response handling [[see example](http://macournoyer.com/blog/2009/06/04/pusher-and-async-with-thin/)]. That way the response would be queued up with all the Data Dog requests and the request from Monit would timeout.

cc @tylerschultz