Whenever a new extension is discovered, a RegisteredExtension is

created in the database, and if one was created by another process since

it was checked, the extension manager would try fetching it. However,

since we're inside a transaction, this could end up aborting it instead

of doing what we want it to do.
We now use get_or_create(), which manages this inside a savepoint. It

does introduce one extra SELECT for each newly-discovered extension,

but that's going to be a rare event. This is still better than

attempting to manage transaction logic ourselves.

Unit tests pass.