Need to schedule compactions when manual compaction finishes

Summary: If there is an outstanding compaction scheduled but at the time a manual compaction is triggered, the manual compaction will preempt. In the end of the manual compaction, we should try to schedule compactions to make sure those preempted ones are not skipped.

Test Plan: make all check

Reviewers: yhchiang, ljin

Reviewed By: ljin

Subscribers: leveldb, dhruba, igor

Differential Revision: https://reviews.facebook.net/D21321
Include candidate files under options.db_log_dir in FindObsoleteFiles()

Summary: In FindObsoleteFiles(), we don't scan db_log_dir. Add it.

Test Plan: make all check

Reviewers: ljin, igor, yhchiang

Reviewed By: yhchiang

Subscribers: leveldb, yhchiang

Differential Revision: https://reviews.facebook.net/D21429
Changes to support unity build:
* Script for building the unity.cc file via Makefile
* Unity executable Makefile target for testing builds
* Source code changes to fix compilation of unity build