Automatically pick an ephemeral port when binding if none is provided

[Benjamin Jaton]

Currently the user has to provide a specific port even when creating a client. The problem is that there is no reliable way to get an available port.

The tests are either:

• hardcoding the ports - the test fails if another service is already using that port
• using NettyMessagingServiceTest#findAvailablePort which is prone to race conditions

More generally when using Atomix clients it is desirable to have Atomix pick a port automatically instead of having to reserve one or resolve one dynamically.

It would be nice to let Netty /InetSocketAddress select an available ephemeral port.
It's possible to achieve this by using Address.fromAddress(0) or Address.fromAddress("host:0").

However there is currently no way to retrieve the port that has been chosen.
I would suggest to expose the resolved / bound port.

I've been experimenting with this in benji@6fe5f6b

Let me know if you think that could be a good change to the project.