This change removes usage of eventlet timers.
This allows coordinator heartbeat/watchers to work correctly
when the main thread is stuck for any reason (IO, time.sleep, ...).
This also fixes a concurrency issue in the notification-agent between
stop/reload_pipeline/refresh_agent that manipulate listeners.
For example a listener stopped by stop() could be restart by
reload_pipeline or refresh_agent. Now we use the coord_lock to
protect the listener manipulations and ensure we are not in a shutdown
process when we restart it.
This bug can't occurs with greenlet because we don't monkeypatch system
call for a while now and all of this methods wasn't ran in concurrency
manner. But remplacing greenlet by reel thread have show up the bug.
Closes-Bug: #1582641
Change-Id: I21c3b953a296316b983114435fcbeba1e29f051e
1ab0acb851