HandlerMapping, HandlerAdapter, HandlerResultHandler (+ HandlerResult)
as the basic request handling contracts. DispatcherHandler to drive
overall request handling.

DispatcherApp provides minimal implementations of the above contracts
enough to put together a running example that returns
200 text/plain "Hello world".

This commit adds RxNetty integration that includes RxNetty-based
implementations of ServerHttpRequest and ServerHttpResponse as well as
an adapter from the RxNetty RequestHandler to the HttpHandler contracts.

Only byte[] is supported at the moment for reading and writing with a
corresponding copy to and from Netty ByteBuf.

This commit introduces HTTP request and response abstractions along
with Servlet-based implementations similar to the ones in the http
package of spring-web but using Reactive Streams.

In turn HttpHandler now accepts the request and response types and
returns Publisher<Void> that reflects the end of handling.

The write method on the response also returns Publisher<Void> allowing
deferred writing. At the moment however the underlying Servlet 3.1
support only supports a single publisher after which the connection
is closed.

Only simple byte[] is supported for reading and writing.

For Tomcat, failures occur if HttpHandlerServlet buffer size is
significantly less than the Tomcat input buffer size.

Also on command line Jetty tests fail with port unavailable unless
the server from the Tomcat tests is also destroyed.