integrate rate limiter into rocksdb

Summary:
Add option and plugin rate limiter for PosixWritableFile. The rate
limiter only applies to flush and compaction. WAL and MANIFEST are
excluded from this enforcement.

Test Plan: db_test

Reviewers: igor, yhchiang, sdong

Reviewed By: sdong

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D19425
report correct interval amplification

Summary: as title

Test Plan: make release

Reviewers: sdong, yhchiang, igor

Reviewed By: igor

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D19515
make rate limiter test more reliable

Summary:
Randomize keys so that compaction actually happens.
Change the config so that compaction happens more aggressively.
The test takes longer time, but the results are more stable shown by
iostat

Test Plan: ran it

Reviewers: igor, yhchiang

Reviewed By: yhchiang

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D19533
fix one more internal_stats issue

Summary: stall count is wrong

Test Plan: make release

Reviewers: sdong, yhchiang, igor

Reviewed By: igor

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D19539