internal/v4, internal/router: use concurrency

This might speed up search results considerably.
Not directly. It's related to the number of bulk hundler types that we can have for any given request. Currently in v4 there are only 3 (I mistakenly thought that the manifest was one of them, but it's also styled as an entity handler, which is not unreasonable, as it does need the entity too). So it does already cover all the possible metadata endpoints. I'll add a comment.