Periodic garbage collection of stats

[dcorbacho]

Although most stats are properly cleaned up after queue/channel/... are deleted, there might be some corner cases missing. It is even possible that a race condition between consuming stats from core tables and deleting objects is happening, as we have seem some very small leaks of channel_created_stats in some deployments.

On top of the current gc strategy, we should implement a periodic gc that verifies that stats belong to a living object on the system. Otherwise, they should be deleted.