Summary: This adds (behing a feature flag) the option for Mercurial servers to record bundles they ingest into a `forwardfillerqueue`. This queue is consumed by the commit cloud forwardfiller (see stack rooted at D15712911 for that) to replay those commits into Mononoke.
Reviewed By: farnz
Differential Revision: D15759638
fbshipit-source-id: 70660343408bbaa865fc8f51f49f590eaf525cce
