[RelDB] Concurrency-related stuff

There seem to be some uncertainties in regards to locking and concurrency. Not necessarily blockers, but uncomfortable things nonetheless. Here's a list of topics:

• See this comment https://github.com/OpenSlides/openslides-backend/issues/3305#issuecomment-3861162976 and the one above.
• test_create_sequential_number.py test_create_sequential_numbers_race_condition problem will need to be investigated (Impulse from @hjanott was to search for dead locks in postgres)
• db_connection_handling.py in create_os_conn_pool, try enabling prepared statements again, according to @hjanott they were disabled due to problems with concurrency.
• system/test_write.py: the two_write_requests_with_locked_fields test deals with locks and is currently skipped. It should be fixed or deleted. It is quite possible that the concept behind the test is obsolete.
• action/test_action_command_format.py: Implement locked fields or delete commented out code. Apparently 95% chance that the test is obsolete.

Start by reviewing the linked comment on issue 3305 and the listed files: test_create_sequential_number.py, db_connection_handling.py, system/test_write.py, and action/test_action_command_format.py. Investigate the concurrency behavior and determine which skipped or commented-out tests and prepared-statement settings remain valid. Done means each listed uncertainty has a documented resolution and the relevant tests or code are updated accordingly.