Properly create xhpast database

Summary:
It isn't deleted by storage destroy.

This should be a no-op on current storage because we execute CREATE DATABASE IF NOT EXISTS.

Test Plan:

$ bin/storage destroy --dryrun

Reviewers: nh, epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D3659