Add graceful exit support to listen_mo_messages command

[tobiasmcnulty]

Still works for smpp_client:

$ python manage.py smpp_client foo
2022-12-19 23:06:46,412 smpp_gateway.queries INFO     Waiting for notifications on channel 'foo'
2022-12-19 23:06:46,413 smpp.Client.140545233456720 INFO     Connecting to 1.2.3.4:2775...
2022-12-19 23:06:46,416 smpp.Client.140545233456720 INFO     Entering main listen loop
^C2022-12-19 23:06:53,511 smpp_gateway.utils   INFO     Got signal 2, setting exit_signal_received = True

2022-12-19 23:06:56,433 smpp.Client.140545233456720 INFO     Got exit signal, leaving listen loop
2022-12-19 23:06:56,433 smpp.Client.140545233456720 INFO     Unbinding...
2022-12-19 23:06:56,435 smpp.Client.140545233456720 INFO     Disconnecting...

Also works for listen_mo_messages:

$ python manage.py listen_mo_messages
2022-12-19 23:07:04,345 smpp_gateway.queries INFO     Waiting for notifications on channel 'new_mo_msg'
^C2022-12-19 23:07:07,244 smpp_gateway.utils   INFO     Got signal 2, setting exit_signal_received = True
2022-12-19 23:07:09,347 smpp_gateway.subscribers INFO     Received exit signal, leaving listen loop...

[Jdsleppy]

Looks good 👍