chore (c sshnpd): remove background thread in favor of synchronous job queuing

[XavierChanth]

Original ticket contents:

chore (c sshnpd): use pthread clean up functions to handle memory management on threads

https://man7.org/linux/man-pages/man3/pthread_cleanup_push.3.html

New contents:

The purpose of this ticket has changed, as the simpler option is to remove multi-threading entirely. The second thread in c sshnpd only runs a periodic job to refresh device info parameters. In order to keep things efficient, this means managing semaphores around the atclient and risking deadlocks.

Once policy has been introduced to c sshnpd, I expect that the median number of device_info entries to refresh with either be 0 or 1. Thus the job can be done synchronously since it will be very quick.

[XavierChanth]

Putting this in the backlog, we need to do this, but not until we are ready to implement RPC, and we won't be ready for another 1-2 sprints.