Extended transactional file locking

We have recently started to lock files for transactional updates in !3821 (merged). This change has been deployed to production without any observed problems, so I have now started to extend the approach to cover all sites known to me where we write to files in repositories. This puts us in a position to better catch concurrent updates of the same file.

Starting with this MR, we should consider every location which updates files without locking and transactional voting a bug that needs to be fixed.