[Reviewer: Andy] Reset event state when we restart a service

This makes grace periods behave properly. Tested by making clearwater-nginx listen on the wrong port, and making sure thre were always three failed polls in the logs between restarts (rather than three failed polls, then a restart on every poll).

Once we take https://github.com/ClearwaterCore/monit/commit/c358150f44ac14c5066a9fe79536dc45739ba2d7 and install this version of monit instead of the main Ubuntu version, this will fix https://github.com/Metaswitch/chef/issues/28.