Connectors and Servers etc. are unclosable from the standalone API

Using e.g. Xnio.createTcpConnector(), there's no way to shut down a TcpConnector after it's been created.

Perhaps Clients and Connectors should be Closeable as well? But this clashes with the microcontainer use case where the container really ought to manage such things.

We could go the same route that the JDK does with Executors and have a ConnectorService, ClientService, TcpConnectorService, TcpClientService which are accessible from the standalone API. These would either be Closeable or they'd have a shutdown()/stop() method.