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