Performance: is_verified system field performance bottleneck

[ntarocco]

The is_verified fetch all members of the community, with a DB query.
This leads to 2 issues:

• if a request like /api/communities, multiple DB query are performed
• it looks like that cls(obj.data, model=obj) in get_members, used by the system fields, triggers an UPDATE query:

File "/usr/local/lib/python3.9/site-packages/invenio_communities/communities/records/systemfields/is_verified.py", line 25, in calculate
    owners = [m.dumps() for m in Member.get_members(record.id) if m.role == "owner"]
  File "/usr/local/lib/python3.9/site-packages/invenio_communities/members/records/api.py", line 153, in get_members
    return [cls(obj.data, model=obj) for obj in q.all()]
  File "/usr/local/lib64/python3.9/site-packages/sqlalchemy/orm/query.py", line 2773, in all
    return self._iter().all()
  File "/usr/local/lib64/python3.9/site-packages/sqlalchemy/orm/query.py", line 2916, in _iter
    result = self.session.execute(
  File "/usr/local/lib64/python3.9/site-packages/sqlalchemy/orm/session.py", line 1665, in execute
    ) = compile_state_cls.orm_pre_session_exec(
  File "/usr/local/lib64/python3.9/site-packages/sqlalchemy/orm/context.py", line 312, in orm_pre_session_exec
    session._autoflush()
  File "/usr/local/lib64/python3.9/site-packages/sqlalchemy/orm/session.py", line 2253, in _autoflush
    self.flush()
  File "/usr/local/lib64/python3.9/site-packages/sqlalchemy/orm/session.py", line 3449, in flush
    self._flush(objects)
  File "/usr/local/lib64/python3.9/site-packages/sqlalchemy/orm/session.py", line 3549, in _flush
    flush_context.execute()
  File "/usr/local/lib64/python3.9/site-packages/sqlalchemy/orm/unitofwork.py", line 456, in execute
    rec.execute(self)
  File "/usr/local/lib64/python3.9/site-packages/sqlalchemy/orm/unitofwork.py", line 630, in execute
    util.preloaded.orm_persistence.save_obj(
  File "/usr/local/lib64/python3.9/site-packages/sqlalchemy/orm/persistence.py", line 237, in save_obj
    _emit_update_statements(
  File "/usr/local/lib64/python3.9/site-packages/sqlalchemy/orm/persistence.py", line 975, in _emit_update_statements

Related fixes:

• models: avoid flushing when getting records invenio-rdm-records#1481
• models: avoid flushing when getting records invenio-drafts-resources#251
• models: avoid flushing when getting records invenio-users-resources#112