Add stack trace to logs, when service cancelled 2x

This can be a sign that some caller isn't properly managing the
cancellation cycle. But... maybe we should just quietly allow double
cancellations? I think not, at least yet, because it still seems to be
uncovering bugs, when we hunt it down.

p2p/service.py

@@ -300,6 +300,7 @@ def _log_tasks(self, message: str) -> None:
     def cancel_nowait(self) -> None:
         if self.is_cancelled:
             self.logger.warning("Tried to cancel %s, but it was already cancelled", self)
+            self.logger.debug("Second cancellation of %s: stack trace", self, stack_info=True)
             return
         elif not self.is_running:
             raise ValidationError("Cannot cancel a service that has not been started")