Summary:
Introduce `RepeatableThread` utility to run task periodically in a separate thread. It is basically the same as the the same class in fbcode, and in addition provide a helper method to let tests mock time and trigger execution one at a time.

We can use this class to replace `TimerQueue` in #4382 and `BlobDB`.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4423

Differential Revision: D10020932

Pulled By: yiwu-arbug

fbshipit-source-id: 3616bef108c39a33c92eedb1256de424b7c04087