[Core] Timeout for killing children in the subprocess supervisor

[mat-hek]

Currently the SubprocessSupervisor, which is responsible for terminating children and utility processes, waits until they all exit. Sometimes they trap exit though and never exit due to a bug, and everything hangs, which is hard to debug. I think there should be a 5-second timeout after which the supervisor force kills the given process. We can make this timeout configurable, but I'm not sure it's needed. The same should probably be done in the PipelineSupervisor