The message id in the CoAP header is used for reliability when
transferring a CoAP PDU over a non-reliable transport protocol
such as UDP. For message de-duplication, it is useful to have
a monotonously increasing number space within a session. Therefore,
the context-global value has been moved to coap_session_t.
The function coap_new_message_id() now takes a coap_session_t
object as its argument.