[5.x] Use cache locks in static cache

[jasonvarga]

This swaps the lock in the Static Cache middleware from a hardcoded file-based Symfony Lock to Laravel's cache-based atomic lock.

This will allow people that use a horizontal scaling approach to use a more appropriate cache driver for locks. e.g. Redis.

This will use the same static_cache cache store if it exists. Docs

This also moves the locking behavior to only around the writable bits. If there's a cached response available to be served, it will be served without waiting for the lock.