MemTable::PostProcess() can skip updating num_deletes if the delta is 0
MemTable::PostProcess() can skip updating num_deletes if the delta is 0

Summary: In many use cases there is no deletes. No need to pay the overhead of atomically updating num_deletes.

Test Plan: Run existing test.

Reviewers: ngbronson, yiwu, andrewkr, igor

Reviewed By: andrewkr

Subscribers: leveldb, andrewkr, dhruba

Differential Revision: https://reviews.facebook.net/D60555

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