Fix cancellation of pending tasks

Formerly, all uncancelled tasks have been treated as if they are done.
This resulted in exceptions when trying to fetch the result (or
exception) from the task for logging purposes.

Now, all tasks are cancelled if they are not already cancelled or not
done.

WIP: Needs test

Author: lgrahl

Diff for: saltyrtc/server/protocol.py

@@ -602,7 +602,11 @@ def _cancel_coroutine_or_task(self, coroutine_or_task, mark_as_done=False):
         else:
             if mark_as_done:
                 coroutine_or_task.add_done_callback(self.task_done)
-            if not coroutine_or_task.cancelled():
+            # Note: We need to check for .cancelled first since a task is also marked
+            #       .done when it is cancelled.
+            if coroutine_or_task.cancelled():
+                self.log.debug('Already cancelled task {}', coroutine_or_task)
+            elif coroutine_or_task.done():
                 exc = coroutine_or_task.exception()
                 if exc is not None:
                     message = 'Ignoring exception of queued task {}: {}'