Support purging logs from separate log directory

Summary:
1. Support purging info logs from a separate paths from DB path. Refactor the codes of generating info log prefixes so that it can be called when generating new files and scanning log directory.
2. Fix the bug of not scanning multiple DB paths (should only impact multiple DB paths)

Test Plan:
Add unit test for generating and parsing info log files
Add end-to-end test in db_test

Reviewers: yhchiang, ljin

Reviewed By: ljin

Subscribers: leveldb, igor, dhruba

Differential Revision: https://reviews.facebook.net/D21801