db_stress to add an option to periodically change background thread pool size.

Summary: Add an option to indicates the variation of background threads. If the flag is not 0, for every 100 milliseconds, adjust thread pool size to a value within the range.

Test Plan: run db_stress

Reviewers: haobo, dhruba, igor, ljin

Reviewed By: ljin

Subscribers: leveldb, nkg-

Differential Revision: https://reviews.facebook.net/D18759