Improve method check_delegator_restriction in delegation_based_restriction_mixin.py

[r-peschke]

First improvement:
Set the call parameter lock_result of the resulting db-calls to False. Reason: The result of the method are part of user permissions. It is not necessary to lock database in this cases.

Second improvement:
Reduce the database actions:

1. Read the meeting_user instances:
   meeting_users = self.datastore.filter(
   "meeting_user",
   And(
   Or(
   FilterOperator("meeting_id", "=", meeting_id)
   for meeting_id in meeting_ids
   ),
   FilterOperator("user_id", "=", self.user_id),
   FilterOperator("vote_delegated_to_id", "!=", None),
   ),
   ["meeting_id"],
   )
2. If there are meeting_users in the above result set, get their meeting_ids
3. Read the meetings from point 2 with a get_many-db-call
4. Get the broken_meetings the way the old implementation does without the meeting_user-related condition