Improve users_build_cache_tree scheduling.

[corentin-soriano]

Avoid user_build_cache_tree duplicates on each folder creation.
Do not start the same task again if the scheduler is launched several times.

An interesting case where the task caused an outage: multiple user_build_cache_trees per user and 455 tasks were performing the same actions at the same time.

[root@localhost ~]# ps aux | grep background_tasks___user_task.php | wc -l
455

Yes, the server has reached 300 of load avg!

[nilsteampassnet]

Very nice.
I already experimented such outage but couldn't fixed it.
👍