Memory pressure under heavy load

[akira]

Based on other people's reports, it seems that there is memory pressure under heavy usage. This was confirmed by doing a test that constantly enqueues maximum tasks to stress the system.

This seems to be caused by 2 issues:

1. The majority of the cause is from stats module - it seems that it falls behind the other parts of the system which causes those messages to queue up.
2. The supervisor for the workers does not seem to be cleaning up properly.

These need to be investigated. The batched stats changes from #230 should fix issue 1.

[akira]

Ran stress test again, and this seems fixed by #265