dynamic soft_rate_limit and hard_rate_limit

Summary: as title

Test Plan:
unit test
I am only able to build the test case for hard_rate_limit.
soft_rate_limit is essentially the same thing as hard_rate_limit

Reviewers: igor, sdong, yhchiang

Reviewed By: yhchiang

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D24759