Fix UniqueConstraintViolationException while insert into oc_file_locks

* fixes #9305 by not being prone to the race condition in insertIfNotExists
* fixes #6899 by not using a query that can result in a deadlock
* replaces the insertIfNotExists call with an insert which is wrapped into a try-catch block
* followup to #12371

Signed-off-by: Morris Jobke <hey@morrisjobke.de>