Test concurrency

[facundominguez]

At a minimum, the TCP transport has a few threads on a couple of nodes which all need to be coordinated to work as expected.

n-t-tcp currently lacks the ability to test different schedules of these threads and ensure that all of them satisfy the properties in tests.

We need to enhance the tests to allow for this. Choose an existing solution for testing concurrency or develop one. Most likely, the code will need to be instrumented or refactored for this sake.