R1:c5e8de945020

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…