Summary:
A simple CLI which calles DB->CompactRange()
Can take String key's as range.

Test Plan:
Inserted data into a table.
Waited for a minute, used compact tool on it. File modification time's
changed so Compact did something on the files.

Existing unit tests work.

Reviewers: heyongqiang, dhruba

Reviewed By: dhruba

Differential Revision: https://reviews.facebook.net/D5697