dns_hub lingers

[Douglas Bagnall]

I find that our newest and simplest dns server still has a few
surprises.

After a number of `make test`s, I end up with several dns_hub.py
processes hanging around (according to `ps faux |grep dns_hub`).

Looking inside, I see:

(gdb) py-bt
Traceback (most recent call first):
  <built-in method acquire of _thread.lock object at remote 0x7f32fb272e18>
  File "/usr/lib/python3.6/threading.py", line 1072, in
_wait_for_tstate_lock
    elif lock.acquire(block, timeout):
  File "/usr/lib/python3.6/threading.py", line 1056, in join
    self._wait_for_tstate_lock()
  File "/usr/lib/python3.6/threading.py", line 1294, in _shutdown
    t.join()

I can look into this eventually if no dns_hub experts leap forward.
Garming thinks it has something to do with hitting control-C.

Douglas