Sending SIGTERM twice tracebacks

Poezio version: e5e0533b
Slixmpp version: slixmpp@2b666eb1

Steps to reproduce

Launch poezio
Send SIGTERM twice in a row.

What happens

Poezio registers the first SIGTERM, starts the shutdown procedure, sees the second SIGTERM, tries to schedule it, and fails.

What I expect

Poezio shutdowns gracefully.

That's probably low priority, because at this point I expect poezio to quit, but there might be things I want to run to completion. Also that might reveal other issues with how events are handled.

Logs.

Traceback (most recent call last):
  File "/usr/bin/poezio", line 11, in <module>
    load_entry_point('poezio==0.13.dev108', 'console_scripts', 'poezio')()
  File "/usr/lib/python3.7/site-packages/poezio/__main__.py", line 8, in run
    main()
  File "/usr/lib/python3.7/site-packages/poezio/poezio.py", line 129, in main
    loop.run_forever()
  File "/usr/lib/python3.7/asyncio/base_events.py", line 539, in run_forever
    self._run_once()
  File "/usr/lib/python3.7/site-packages/poezio/asyncio.py", line 27, in my_run_once
    real_run_once(self)
  File "/usr/lib/python3.7/asyncio/base_events.py", line 1739, in _run_once
    event_list = self._selector.select(timeout)
  File "/usr/lib/python3.7/selectors.py", line 468, in select
    fd_event_list = self._selector.poll(timeout, max_ev)
  File "/usr/lib/python3.7/site-packages/poezio/core/core.py", line 506, in exit_from_signal
    self.disconnect('%s received' % signals.get(sig))
  File "/usr/lib/python3.7/site-packages/poezio/core/core.py", line 837, in disconnect
    self.xmpp.disconnect()
  File "/home/ppjet/Src/slixmpp/master/slixmpp/xmlstream/xmlstream.py", line 473, in disconnect
    self.abort, repeat=False)
  File "/home/ppjet/Src/slixmpp/master/slixmpp/xmlstream/xmlstream.py", line 826, in schedule
    "There is already a scheduled event of name: %s" % name)
ValueError: There is already a scheduled event of name: Disconnect wait