vici: Make threads handling requests get canceled explicitly

vici: Make threads handling requests get canceled explicitly

Threads initiating SAs can get stuck on the semaphore in
wait_for_listener() during shutdown if the corresponding job is never
executed.  A particular case when this can happen is if more initiations
are triggered than worker threads are available.  This causes a (known)
deadlock as no workers are free anymore to process jobs (for inbound
messages or timeouts etc.), including the one to initiate an SA.
This change at least allows a proper shutdown.