Make bin/storage dump insert CREATE DATABASE and USE statements

Summary:
Ref T13000. The new approach for dumping database-by-database means that we don't get CREATE DATABASE or USE statements, which makes importing the dump again inconvenient.

Manually stitch these into the dump.

Test Plan:

• Used bin/storage dump --namespace ... to dump a smaller local instance.
• Used bin/storage destroy --namespace ..., to destroy the namespace, then inported the dump cleanly.
• Verified that each CREATE DATABASE statement appears only once.
• Verified that bin/storage renamespace --live can correctly process this file.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13000

Differential Revision: https://secure.phabricator.com/D18707