Having a distinct class just for that was a bit overkill

Rename classes to (hopefully) clearer names while we're doing that.

Remove superfluous lib: true, type: redis, service: true, models: true, services: true, no_db: true, api: true

Signed-off-by: Rémy Coutable <remy@rymai.me>

Use `update_all` to only require one query per discussion to
update the notes resolved status. Some changes had to be made to
the discussion spec to accout for the fact that notes are not
individually updated now

Speedup DiffNote#active? on discussions, preloading noteables and avoid touching git repository to return diff_refs when possible

- Preloading noteable we share the same noteable instance when more than one 
discussion refers to the same noteable.
- Any other call to that object that is cached in that object will be for any 
discussion.
- In those cases where merge_request_diff has all the sha stored to build a diff_refs get that 
diff_refs using directly those sha instead accessing to the git repository to first get the 
commits and later the sha.