Use message token to match request and response, remove token from empty message

[snorrihann]

According to https://datatracker.ietf.org/doc/html/rfc7252

In 4.1. Messages and Endpoints
An Empty message has the Code field set to 0.00. The Token Length
field MUST be set to 0 and bytes of data MUST NOT be present after
the Message ID field. If there are any bytes, they MUST be processed
as a message format error

In 5.2. Responses
After receiving and interpreting a request, a server responds with a
CoAP response that is matched to the request by means of a client-
generated token (Section 5.3); note that this is different from the
Message ID that matches a Confirmable message to its Acknowledgement.

[StFS]

Okay, this is actually quite amazing!

I just ran the unit tests for CoAPthon3 in the following configurations:

• The master branch of the original project
• The master branch of the controlant-org project
• The master branch of the snorrihann project (which this PR is based off)

Now get this... the unit tests hang in the first two projects on a test called test_separate!?! On the bugfix branch, that test passes!

So it seems that the unit tests that already existed were actually catching this bug.

There is a unit test that fails though (test_not_found) but that is failing on all 3 branches so that's probably not our fault.

I took a look at our GitLab pipelines that have been running these unit tests since we forked the project (https://gitlab.controlant.com/device-platform/libraries/third-party-libraries/CoAPthon3/-/pipelines) and can verify that all runs there have timed out on test_separate. I found only one test run in GitLab that had attempted the test_not_found test and that failed (https://gitlab.controlant.com/device-platform/libraries/third-party-libraries/CoAPthon3/-/jobs/1865322) so I think that's completely unrelated to this change.