rsc stop: assert that things are shut down + attempt again, if not

[HenrikBengtsson]

We're currently signal SIGTERM to all processes we want to shut down:

https://github.com/UCSF-CBI/rstudio-server-controller/blob/e1416dcb6b2ebf8a837eb50141dd20cd875463b3/bin/rsc#L1369-L1396

but we don't really make sure the processes are truly terminated afterward. It could be that they keep running, e.g. stuck or overwhelmed processes.

Actions

• [x] Use already existing terminate_rsession(), terminate_rserver(), terminate_rserver_monitor(), and terminate_ssh_revtunnel().
• [x] Assert that processes are terminated afterward. If not, produce a warning, error, or escalate to SIGKILL or SIGQUIT?
• [x] Give processes some time to terminate, before checking.

[HenrikBengtsson]

Now tries multiple times with TERM, sleep one second before checking, and if still running, sleep two seconds and repeat. After four failed TERMs, do one last with KILL.