Symfony version(s) affected: all

Description
The FlockStore does not create the directory if it does not exist.
I don't know if it is made on purpose...

When declaring the lock system with framework bundle:

# /packages/framework.yaml
framework:
    lock:
        file: 'flock://%kernel.project_dir%/var/lock'

This results in The directory "/path/to/var/lock" is not writable. althought the ./var directory is writable.
Why don't we check if the directory exists, and create it if it doesn't?

How to reproduce
Declare the lock with the given yaml, when directory ./var/lock does not exist.

Possible Solution
Create the directory if it doesn't exist, in the FlockStore constructor