Add filter to allow cache locking to be disabled.

See websharks/quick-cache#387
Move `find_wp_config_file()` to share class.

See https://github.com/websharks/quick-cache/commit/5c95714
Add new filter to allow site owner to specify cache locking method.

See websharks/quick-cache#387
Improve new cache lock filter names as per Jason's feedback.

See websharks/quick-cache#388
Merge branch 'feature/387' into 000000-dev