With track_client_session set, an incoming session gets its own proxy
entry for each upstream server it uses. coap_proxy_remove_association()
returned after the first entry tracking the session, so the proxy
requests of that session held in later entries were left in place when
the session was deleted, and their upstream sessions were not released.
A later upstream response was then forwarded to the freed session.
Carry on with the remaining entries instead of returning.
Add a unit test.
Alejandro Pacheco
85ed6ebb93