FileBakend have some race condition
One of the problem is that get() can be called before the file is available, so we need multiple try of file_get_contents like we have in set() for the rename. In the patch, I add a usleep, really short, to increase the wait time a bit between each try.
[TASK] Add support for file locking in (Simple)FileBackend
With concurrent access the current FileBackend and SimpleFileBackend can
be racy, this patch adds a strict file locking when writing and shared
locking when ready to avoid this problem.