R1:087cc0808a34

Make SQL patch management DAG-based and provide namespace support

Summary:
This addresses three issues with the current patch management system:

# Two people developing at the same time often pick the same SQL patch number, and then have to go rename it. The system catches this, but it's silly.
# Second/third-party developers can't use the same system to manage auxiliary storage they may want to add.
# There's no way to build mock databases for unit tests that need to do reads.

To resolve these…