Summary:
The method DeleteObsolete files is a very costly methind, especially
when the number of files in a system is large. It makes a list of
all live-files and then scans the directory to compute the diff.
By default, this method is executed after every compaction run.

This patch makes it such that DeleteObsolete files is never
invoked twice within a configured period.

Test Plan: run all unit tests

Reviewers: heyongqiang, MarkCallaghan

Reviewed By: MarkCallaghan

Differential Revision: https://reviews.facebook.net/D6045