Runtime: fix potentially missing events when multithreading.

Fix a bug that when a DistAlgo process has multiple threads, events that
are label-restricted might get dropped due to potential race condition.
Also catch type errors when calling handlers.

* da/sim.py(DistProcess._process_jobqueue): don't assign new deque
  instance to 'self._jobqueue', extend it; catch TypeError for handler
  calls(most likely due to mismatched arguments) and log an error
  message.
Runtime: 'import_da' no longer an api function.

'import_da' is likely to be used at the global level and should not
assume the 'da' system is initialized.

* da/__init__.py: add 'api.import_da' to '__all__'.

* da/api.py: remove @api decorator for 'import_da'.