Rate limiter for BackupableDB

Summary: Might be useful if client doesn't want to effect running system during backup too much.

Test Plan: added a test case

Reviewers: dhruba, haobo, ljin

Reviewed By: haobo

CC: leveldb

Differential Revision: https://reviews.facebook.net/D17091
Relax backupable RateLimiter unit test for slow environments