Summary:
Add a read option `background_purge_on_iterator_cleanup` to avoid deleting files in foreground when destroying iterators.
Instead, a job is scheduled in high priority queue and would be executed in a separate background thread.

Test Plan: Add a variant of PurgeObsoleteFileTest. Turn on background purge option in the new test, and use sleeping task to ensure files are deleted in background.

Reviewers: IslamAbdelRahman, sdong

Reviewed By: IslamAbdelRahman

Subscribers: andrewkr, dhruba

Differential Revision: https://reviews.facebook.net/D59499