Refactor of TCP transport.

[NiteshKant]

Problem

This is a split of PR #106 to make local transport use TCP.
There seems to be some issue in netty's local transport which creates race-conditions and causes test failures.
These refactor has some changes that are not necessarily related to local transport but makes the usage better.

Modification

Refactored common test classes to reactivesocket-test module so they can be used in both tcp and local transport without code duplication.
Added PayloadImpl as a utility implementation of Payload.

[NiteshKant]

Since this is a subset of PR #106 which got a 👍 I am just merging this now.

[yschimke]

Are you sure the issues for Local Transport were not because you were reusing the same local DuplexConnection over all tests?

I haven't noticed any issues so far but I was using the test name in the config for each test, so never reusing connections.

[NiteshKant]

@yschimke ClientSetupRule reconnects to a new socket (new ID generated randomly) for every test. The code is here and here

So, it doesn't use the same connection AFAICT.

[yschimke]

I was looking at the tests before changes, but might be the wrong tests :(

public class ClientServerTest {

    static ReactiveSocket client;

    static ReactiveSocket server;

    @BeforeClass
    public static void setup() throws Exception {
        LocalServerReactiveSocketConnector.Config serverConfig = new LocalServerReactiveSocketConnector.Config("test", new ConnectionSetupHandler() {
            @Override
            public RequestHandler apply(ConnectionSetupPayload setupPayload, ReactiveSocket rs) throws SetupException {
                return new RequestHandler() {
                    @Override