The extension manager now has full support for fine-grained loading from

database changes. Local extension state is updated to reflect database

state, meaning the extension manager no longer needs to shut down

everything and bring it all back up. This should improve performance

across the product, especially for extensions that may need to record

state in the database.
This change puts in the final pieces of the fine-grained loading

process and switches the middleware to call a plain load() instead of

a full-reload when the sync generation changes.
There were some small changes to the process that were needed. Load

errors are now cleared at the start of the process, so that there's no

risk of stale load errors sticking around if extensions disappear. If an

existing extension still fails to load, it will just store a new load

error anyway.
Template caches are also now cleared at the end, if extension state

changed.
The "Reload Extensions" button in the Extensions page will continue to

do a full reload.

Unit tests passed.
Smoke-tested this, with enabling/disabling/changing extension state

and seeing it reflected.
Haven't hit any issues, but needs more testing in production.