Functionality to flush the cache for a domain or any other batcache key

[xdancho]

Flush keys can be used to change the cache key hash by adding an additional element to the $batcache->keys array. This can be used to flush the cache for any key that can have duplicates, e.g. host.
For example, one may want to invalidate the cache for a host/domain via mu-plugin on theme change.

Configuration:

• Requires a global object cache group.
• Add a key from $batcache->keys in $flush_keys array. For example:

$flush_keys = array( 'host' );

Note: Adding a key in $flush_keys will invalidate the entire batcache cache.

• Call the flush method with args key and the value of that key. Example:
  If we want to flush the cache for a specific host/domain on theme change:

add_action ( 'switch_theme', function(){
    global $batcache;
    $batcache->flush( 'host', $_SERVER['HTTP_HOST'] );
} );

[aidvu]

Nice, thanks for working on this!