Rename Server to HttpServer to avoid class name collisions and ambiguities

[clue]

This changeset renames the Server class to HttpServer to avoid class name collisions and ambiguities.

// deprecated$server = newReact\Http\Server($handler);
$server->listen(newReact\Socket\Server($port));
// new$http = newReact\Http\HttpServer($handler);
$http->listen(newReact\Socket\Server($port));

The deprecated Server is now an alias for new HttpServer to ensure existing code continues to work as-is.