Summary:
Added 2 statistics in compaction job statistics, to
identify if single deletes are not meeting a matching key
(fallthrough) or single deletes are meeting a merge, delete or
another single delete (i.e. not the expected case of put).

Test Plan: Tested the statistics using write_stress and compaction_job_stats_test

Reviewers: sdong

Reviewed By: sdong

Subscribers: andrewkr, dhruba

Differential Revision: https://reviews.facebook.net/D61749