[chore] various federatingdb tweaks (#4178)

after seeing a potential reported federating worker lockup i decided to start digging into the federatingdb code. this PR encompasses:
- removes one of our last unused interface types `federatingdb.DB{}`, replacing it with a struct type `*federatingdb.DB{}`
- in `transport.dereferenceLocal()` differentiates between an unsupported lookup type and ErrNoEntries to reduce unnecessary calls, and reduce potential lockups that may occur while trying to call our own endpoints that then call `federatingdb.Lock()`
- removes a bunch of the locks on follow state changes since the DB already synchronizes that
- removes the unnecessary `pub.Clock{}` struct field and type passed to the transport controller

frankly it would be great if we could remove the locking in `federatingdb.Lock()` and instead handle it ourselves as it gets very confusing trying to figure out what functions will have locks held. but i guess that's one for when we move further away from the go-fed/activity/pub package usage.

Reviewed-on: https://codeberg.org/superseriousbusiness/gotosocial/pulls/4178
Co-authored-by: kim <grufwub@gmail.com>
Co-committed-by: kim <grufwub@gmail.com>

internal/federation/federatingdb/exists.go

@@ -24,6 +24,6 @@ import (
 
 // Exists is an implementation of pub.Database{}.Exists(), optimized specifically for
 // the only usecase in which go-fed/activity/pub actually calls it. Do not use otherwise!
-func (f *federatingDB) Exists(ctx context.Context, id *url.URL) (exists bool, err error) {
+func (f *DB) Exists(ctx context.Context, id *url.URL) (exists bool, err error) {
 	return f.activityIDs.Has(id.String()), nil
 }