Styx server object support.

[mikkokar]

Description

Introduces a new InetServer interface that is a StyxService with a server InetAddress. It is an Internet server that accepts incoming connections on a specified port. The interface is not limited to HTTP. It can be anything as long as it opens a server port.

Because InetServer is a StyxService, it can be started and stopped with the usual Styx service framework.

Extend the Styx configuration object model to accommodate these new InetSerever objects. This allows used to configure styx server objects like so:

servers:
  myHttpServer:
    type: HttpServer
    config:
      port: 8080
      tlsSettings:
       ...

This PR doesn't yet provide any server implementations. Therefore the above configuration wouldn't work against this PR. This PR just extends the configuration framework into which new server implementations can be added.

Open Issues

• The InetServer interface should really be called StyxServer. But this name is already taken, and changing it would break some existing consumers.

• Styx services must be converted to Guava Services before they can be started. This is really inconvenient at times (in tests only), as you need two separate objects. One for the styx service to read the port number, another for starting/stopping the service.

• NettyServer service name.