hdfs cleanup and compile test against CDH 4.4.
Make RateLimiting unit test less flakey
- hdfs cleanup; fix to NewDirectory to comply with definition in env.h
- fix compile error with env_test; static casts added
cleanup exception text
Merge pull request #154 from mrsqueeze/master

hdfs cleanup and compile test against CDH 4.4.
Print pthread_t in a more safe way
Merge pull request #156 from Chilledheart/print_pthread_info

Print pthread_t in a more safe way
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