[8.x] Ability to specify a separate lock connection

[paras-malhotra]

Motivation

This PR allows us to specify a separate lock connection for the Redis and Database cache stores. Related: laravel/ideas#2404.

To use a separate lock connection, we may specify a lock_connection in our config/cache.php like so:

    'stores' => [
        'database' => [
            'driver' => 'database',
            'table' => 'cache',
            'connection' => null,
            'lock_connection' => 'lock',
        ],
        'redis' => [
            'driver' => 'redis',
            'connection' => 'cache',
            'lock_connection' => 'lock',
        ],
    ]

Benefits

The Laravel Framework currently uses locks for several things:

1. The WithoutOverlapping middleware to avoid overlapping jobs
2. Unique jobs
3. Session blocking

Plus of course, locks may also be used in userland for application specific use cases.

The current lock connection is always the same as the cache connection. So, a php artisan cache:clear will also clear your locks. It's absolutely fine to clear cache in production and in fact, even a common deployment step. But clearing locks could mean breaking your unique jobs or session blocking. This PR introduces the ability to have a separate lock connection to make it possible to clear the cache without clearing locks.

There are no breaking changes with this PR that's why it targets 8.x.