Summary:
When creating a database backup, the background threads will not only consume IO resources by copying files, but also consuming CPU such as by computing checksums. During peak times, the CPU consumption by the background threads might affect online queries.

This PR makes it possible to decrease CPU priority of these threads when creating a new backup.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6602

Test Plan: make check

Reviewed By: siying, zhichao-cao

Differential Revision: D20683216

Pulled By: cheng-chang

fbshipit-source-id: 9978b9ed9488e8ce135e90ca083e5b4b7221fd84