Defragmenter: proper warnings on leakage

It looks like we're safe based on stress tests, but we should still
report that condition correctly if it ever happens.
Defragmenter: no null handling for cache removals

See https://github.com/airbnb/plog/pull/71#issuecomment-42583027 for the reasoning.
Merge pull request #71 from airbnb/pierre/fix/defragmenter_warning

Defragmenter: proper warnings on leakage