Flat combining: Update statistics.
Flat combining: Update statistics.

Summary:
Modified FC stats. Added number of sessions and removed the number of out-of-memory record allocations.
Removed locking from reading stats to avoid deadlock if the lock is already held. Reading stats now assumes exclusive access.

Reviewed By: djwatson

Differential Revision: D4857132

fbshipit-source-id: 81e4f25040af3691f3e82fe3794ee72c7ff53a99

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