Clean up duplicate MID handling, and just send back last response if MID
matches.
Removed COAP_BLOCK_CACHE_RESPONSE as no longer required for duplicate MID
handling, and so no need to send an Empty ACK response for a subsequent
Separate response as a referenced copy of the last PDU respponse is now kept
for a session.
Note that if a request is sent by a proxy to an upstream server, this always
converts the upstream server's response into a Separate response back to the
client as the upstream server may take time to respond, do any error recovery
etc.
See https://core-wg.github.io/oscore/test-spec5.html
Add in new test server oscore-interop-server to provide the necessary
functionality.
Add in script and configuration files to run the tests locally.