Added array_replace_recursive merge strategy

Added array_replace_recursive merge strategy

For extension registry, add array_replace_recursive merge strategy,
as some extensions/configuration may prefer that to array_merge_recursive.

In some cases, configuration is merged from multiple extensions,
such as JsonConfig's $wgJsonConfigs configuration: ZeroBanner defines

	"JsonZeroConfig": {
		"namespace": 480,
		"nsName": "Zero",
		"isLocal": false,
		"cacheKey": "1"
	}

and mobile.php overrides it with
	$wgJsonConfigs['JsonZeroConfig']['isLocal'] = false;
	$wgJsonConfigs['JsonZeroConfig']['remote'] = [
		'url' => 'https://zero.wikimedia.org/w/api.php',
		'username' => $wmgZeroPortalApiUserName,
		'password' => $wmgZeroPortalApiPassword,
	];

Having identical value 'isLocal' would be converted into an array
if array_merge_recursive is used, but the replace strategy fixes it.

Change-Id: Ica6ddd0ae76f23e60de9b6235c6e2a3f2754a95d