Fix bug in background sending
Fix bug in background sending

Thread's daemon property needs to be set from calling thread.
Ensure thread is alive whenever we add an event to the queue (for some reason, this is not the case for the first event when this is deployed in a swift proxy setup).

Change-Id: I9a8fad8036a0887653d2ad5f99f6b5b1b953c2db

openstack/ceilometermiddleware