Draft, broken implementation keeping track of in-flight diag requests.
Swap another list in for the hashmap - still broken.
Use SAFE versions of queue.h functions, allows removal during traversal.
Don't blocking sending because of a diag message itself.
Clean up diag messages more often, regardless of tx/rx.
Remove unused constant.
Wait until diag logic decides to send request, rather than immediately.
Resolve to have diag message clean up happening each time through loop.