Got unexpectedly far today on optimising the database that v6 repositories
use to keep track of unlocked files. The database schema may still need
optimization, but everything else to do with the database is optimised.
Writes to the database are queued together. And reads to the database
avoid creating the database if it doesn't exist yet. Which means v5
repos, and v6 repos with no unlocked files will avoid any database
overhead.
