Delete stale fixtures with bin/storage destroy

Summary:
When a PhabricatorTestCase dies after creating storage fixtures, it leaves
those storage fixtures around. This doesn't happen often, but when it does
happen it's a pain to cleanup. The --unittest-fixtures option helps automate
that cleanup.

Test Plan:
Ran it with --dryrun, then for real. Became overwhelmed with a Zen like peace
after regarding the tidiness and beauty of SHOW DATABASES.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D3858