Clean up quitting behavior and add quit trigger.

The closing of Prometheus now using a sync.Once wrapper to prevent
any accidental multiple invocations of it, which could trigger
corruption or a race condition.  The shutdown process is made more
verbose through logging.

A not-enabled by default web handler has been provided to trigger a
remote shutdown if requested for debugging purposes.

Change-Id: If4fee75196bbff1fb1e4a4ef7e1cfa53fef88f2e