Speed up GET /api/v3/site endpoint

[Nutomic]

Adds two commits:

• Make db queries for GET /api/v3/site in parallel using try_join. This required changes to db methods to avoid overlapping &mut borrows of the same value. Luckily these methods are only used in few other places.
• Cache site response. Most of the data in GetSiteResponse is independent from the user account, so we can easily cache it across requests.

cc @dullbananas

[kroese]

Great!! But in my case all the slowdown is caused by a single line:

let admins = PersonView::admins(&mut context.pool()).await?;

which is untouched in your commit.

Do you have any clues why something as simple as retrieving the admins would take almost a full second?

[Nutomic]

Here it would be much better to read all this data in a single SQL query, but it also seems harder to implement.

[dullbananas]

Instead of changing the functions to use ActualDbPool, you can replace this with:

lemmy_db_schema::try_join_with_pool!(pool => (
       |pool| CommunityFollowerView::for_person(pool, person_id),
       |pool| CommunityBlockView::for_person(pool, person_id),
       |pool| InstanceBlockView::for_person(pool, person_id),
       |pool| PersonBlockView::for_person(pool, person_id),
       |pool| CommunityModeratorView::for_person(pool, person_id),
       |pool| LocalUserLanguage::read(pool, local_user_id)
     ))

Pool variable must be defined first because the macro only accepts an identifier:

let pool = context.pool();

The queries for building site_response can also run in parallel.

[Nutomic]

Nice I didnt see that macro, works perfectly.