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
$server = new React\Http\Server($handler);
$server->listen(new React\Socket\Server($port));

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

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