cubicweb #4458094 update_feeds looping task won't stop upon instance shutdown [open]

Hitting CTRL+C on a running instance (debug mode) in the middle of a datafeed import (triggered by update_feeds looping task) does not work.

Basically, I see: `` (cubicweb.repository) INFO: canceling task update_feeds...`` and then a series of tracebacks like:

Exception in thread Thread-7-update_feeds[60]:
Traceback (most recent call last):
  File "/usr/lib/python2.7/threading.py", line 552, in __bootstrap_inner
    self.run()
  File "/usr/lib/python2.7/threading.py", line 760, in run
    self.function(*self.args, **self.kwargs)
  File "/usr/lib/python2.7/dist-packages/cubicweb/server/utils.py", line 132, in auto_restart_func
    func(*args)
  File "/usr/lib/python2.7/dist-packages/cubicweb/hooks/__init__.py", line 60, in update_feeds
    with repo.internal_cnx() as cnx:
  File "/usr/lib/python2.7/contextlib.py", line 17, in __enter__
    return self.gen.next()
  File "/usr/lib/python2.7/dist-packages/cubicweb/server/repository.py", line 923, in internal_cnx
    with cnx.ensure_cnx_set:
  File "/usr/lib/python2.7/contextlib.py", line 17, in __enter__
    return self.gen.next()
  File "/usr/lib/python2.7/dist-packages/cubicweb/server/session.py", line 651, in ensure_cnx_set
    self._set_cnxset()
  File "/usr/lib/python2.7/dist-packages/cubicweb/server/session.py", line 381, in check_open
    return func(cnx, *args, **kwargs)
  File "/usr/lib/python2.7/dist-packages/cubicweb/server/session.py", line 610, in _set_cnxset
    cnxset = self.repo._get_cnxset()
  File "/usr/lib/python2.7/dist-packages/cubicweb/server/repository.py", line 395, in _get_cnxset
    raise Exception('no connections set available after 5 secs, probably either a '
Exception: no connections set available after 5 secs, probably either a bug in code (too many uncommited/rolled back connections) or too much load on the server (in which case you can try to set a bigger connections pool size)