Fix UniqueConstraintViolationException while insert into oc_filecache

* fixes #6160 by not being prone to the race condition in insertIfNotExists
* fixes #12228 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

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