Follows #405
Replaces #412. Cherry-picked over in order to fix & run unit tests.
Introduced in https://github.com/pantheon-systems/wp-redis/pull/405.
wp_cache_flush_runtime should not clear redis cache as well, just local runtime cache values. By passing false to flush method, only runtime caches are flushed.
Follows #405 Replaces #412. Cherry-picked over in order to fix & run unit tests.
Props @spacedmonkey for the fix.