Extension loading would, at the worst case, attempt a fetch + create for

every extension. That create was only necessary the first time an

extension was loaded, and the rest of the time we'd just be doing a

fetch.
Doing one fetch per extension is a waste of time. Instead, we want to

fetch them all up-front, ideally.
This new code keeps track of what we need to fetch and what we need to

create. It then fetches all in one go, and then individually creates any

remaining registere extension entries we may need.

Played around with loading extensions, enabling, disabling, and doing

full reloads. I also deleted some registrations and reloaded the list

of extensions, and saw them populate again.
Unit tests pass.