Reduce storage reads, especially for map storage

[ahus1]

Description

When analyzing #14944, I found that there are a lot of reads for the realm and the clients when accessing the admin UI.

For this issue, assess some low-hanging fruits there, especially:

• don't retrieve the admin realm to just find out if the current realm is the admin realm.
• when iterating over permissions related to a client, retrieve the client once and then check all permissions for the client instead of retrieving the client for every single permission

Motivation

Optimize performance, especially for JPA map storage as each of those queries will (1) run a dirty check, and (2) will hit the DB to retrieve the record