Race condition in \Drupal\content_lock\ContentLock\ContentLock::locking() as we fetch the lock (select) and then separately create the lock (merge)

We can't do the merge idea because merge is implemented as a select and update or insert... so yet another opportunity for races... so we can consider using upsert or using the lock system.

I've tried to write a test for this - both low level and and high level - using pcntl_fork() to simultaneously lock via the API and UI neither worked because it is hard to have multiple sessions as the same time in FunctionalTests and separate service instances in kernel tests.