WL#13378: Component service to add "admin" session

Affects: Server-8.0   —   Status: Complete

It is necessary for internal consumers of the sql session plugin service not to
be a subject of the max_connections limit. 

E.g. group replication can't operate when the server hit max_connections limit.
This is reported as BUG#94987 Add admin connections (not considered on 
max_connections) to SQL API

This worklog is about adding a new component service to allow its caller to
create a special kind of SQL session that is not a subject to the max_connection
limit. And of course to make the GR plugin use this new session instead of the 
regular one that it uses now.