coap-client.c: Cleanup following observe cancellation

Delay exit briefly checking if there are any response packets following
observe cancellation and process as appropriate.

If multiple observes are outstanding, cancelling them using CON will
cause issues as NSTART limit is hit.  The server then does not know
about all of the cancellations.

Also, this can allow the server to properly shutdown observe
activity when blocks are used.

examples/coap-client.c

@@ -1826,6 +1826,7 @@ main(int argc, char **argv) {
           for (i = 0; i < tracked_tokens_count; i++) {
             if (tracked_tokens[i].observe) {
               coap_cancel_observe(session, tracked_tokens[i].token, msgtype);
+              coap_io_process(ctx, COAP_IO_NO_WAIT);
             }
           }
           doing_observe = 0;
@@ -1834,6 +1835,10 @@ main(int argc, char **argv) {
           /* make sure that the obs timer does not fire again */
           obs_ms = 0;
           obs_seconds = 0;
+          for (i = 0; i < 5 ; i++) {
+            /* Make sure all is flushed out */
+            coap_io_process(ctx, 100);
+          }
         } else {
           obs_ms -= result;
         }