Implement a lock backend

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This issue has been tagged with Needs reroll because the patch needed to be re-written to use functions exposed from the APCu extension, not from the APC extension, which is no longer developed nor supported.

There are also other changes that need to be done.
For example, there is probably no need to keep a list of locks as global variable, and there is probably no need to have a shutdown callback to remove all the locks that are still not expired. APCu associates a TTL to its values; after the TTL is passed, the value is automatically purged.

Actually, without editing the settings.php file, it is not possible to set a different lock back-end.
When tests are executed, Drupal core already loaded the includes/lock.inc file. Loading another file that defines the same functions contained in that file, as required to implement a lock back-end, causes errors like Cannot redeclare lock_initialize().