Summary:
Rocksdb backup and restore rate limiting is currently done per backup/restore.
So, it is difficult to control rate across multiple backup/restores. With this
change, a throttler can be provided. If a throttler is provided, it is used.
Otherwise, a new throttler is created based on the actual rate limits specified
in the options.
Test Plan: Added unit tests
Reviewers: ldemailly, andrewkr, sdong
Reviewed By: andrewkr
Subscribers: igor, yiwu, andrewkr, dhruba
Differential Revision: https://reviews.facebook.net/D56265
1147e5b05a