Summary:
Although added to db_stress, testing of backup/restore
was never integrated into the crash test, originally concerned about
performance. I've enabled it now and to address the peformance concern,
testing backup/restore is always skipped once the db exceeds a certain
size threshold, default 100MB. This should provide sufficient
opportunity for testing BackupEngine without bogging down everything
else with heavier and heavier operations.

Also fixed backup/restore in db_stress by making sure PurgeOldBackups
can remove manifest files, which are normally kept around for db_stress.

Added more coverage of backup options, and up to three backups being
saved in one backup directory (in some cases).

Pull Request resolved: https://github.com/facebook/rocksdb/pull/7348

Test Plan:
ran 'make blackbox_crash_test' for a while, with heightened
probabilitly of taking backups (1/10k). Also confirmed with some debug
output that the code is being covered, TestBackupRestore only takes
a few seconds to complete when triggered, and even at 1/10k and ~50MB
database, there's <,~ 1 thread testing backups at any time.

Reviewed By: ajkr

Differential Revision: D23510835

Pulled By: pdillinger

fbshipit-source-id: b6b8735591808141f81f10773ac31634cf03b6c0