Summary:
Worker thread will occasionally call PauseBackgroundWork(),
briefly sleep (to avoid stalling itself) and then call
ContinueBackgroundWork().
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6148

Test Plan:
some running of 'make blackbox_crash_test' with temporary
printf output to confirm code occasionally reached.

Differential Revision: D18913886

Pulled By: pdillinger

fbshipit-source-id: ae9356a803390929f3165dfb6a00194692ba92be