FIFO compaction style

Summary:
Introducing new compaction style -- FIFO.

FIFO compaction style has write amplification of 1 (+1 for WAL) and it deletes the oldest files when the total DB size exceeds pre-configured values.

FIFO compaction style is suited for storing high-frequency event logs.

Test Plan: Added a unit test

Reviewers: dhruba, haobo, sdong

Reviewed By: dhruba

Subscribers: alberts, leveldb

Differential Revision: https://reviews.facebook.net/D18765
Update HISTORY.md
Run FIFO compaction as part of db_crashtest2.py

Summary: As title

Test Plan: ran it

Reviewers: dhruba

Reviewed By: dhruba

Differential Revision: https://reviews.facebook.net/D18783