Part of what makes this work is the test implementation of TransportManager.
This PR is pretty straightforward. A few gotchas though:
* A message signing test was moved into `JacksonTests` because that is
where the signing actually happens.
* A CRDT serializing test was moved there as well. It's the best place
for now.
* No UDP tests at all. I plan to fix that in a bit. Reasoning is that it is
difficult to test any TransportManager implementation without bring up
a full stack. I plan to address this in the future (GOSSIP-83).
* Simple round trip Jackson serialization tests.
