Can't shutdown Raiden if there is a pending transfer

[istankovic]

I tried interrupting (Ctrl-C) my Raiden instance while I had a pending transfer and could not get the instance to exit in a reasonable amount of time:

2021-06-08 09:58:07.431396 [info     ] Signal received. Shutting down. [raiden.ui.runners] signal=<Signals.SIGINT: 2>                                                                         
2021-06-08 09:58:08.101399 [error    ] Error processing request       [raiden.api.rest_utils] errors=Raiden API is shutting down status_code=<HTTPStatus.SERVICE_UNAVAILABLE: 503>            
2021-06-08 09:58:08.102373 [info     ] 127.0.0.1 - - [2021-06-08 11:58:08] "GET /api/v1/payments?offset=992 HTTP/1.1" 503 194 0.001062 [raiden.api.rest.pywsgi]                               
2021-06-08 09:58:08.103533 [error    ] Error processing request       [raiden.api.rest_utils] errors=Raiden API is shutting down status_code=<HTTPStatus.SERVICE_UNAVAILABLE: 503>            
2021-06-08 09:58:08.104654 [info     ] 127.0.0.1 - - [2021-06-08 11:58:08] "GET /api/v1/payments HTTP/1.1" 503 194 0.001150 [raiden.api.rest.pywsgi]                                          
2021-06-08 09:58:08.434059 [info     ] 127.0.0.1 - - [2021-06-08 11:58:08] "POST /api/v1/payments/0x6B175474E89094C44Da98b954EedeAC495271d0F/0x2ac648Bc29F9b9FCdd235433B8dACd6E42cCF1c9 HTTP/1
.1" 500 140 752.266318 [raiden.api.rest.pywsgi]                                                                                                                                               
2021-06-08 09:58:26.039109 [info     ] Synchronizing blockchain events [raiden.raiden_service] blocks_per_second=0.032599961251582564 elapsed=30.67488308598695 greenlet_name=AlarmTask._run n
ode:0x8c31290358f6855bAf060e6FA7b11517444641e3 remaining_blocks_to_sync=0 to_block=12593159