python: Generalize scr watchdog

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The current watchdog implementation is specific to SLURM. It assumes the MPI job was launched with ``srun``, and it queries for the corresponding jobstep id. Then when it detects the job is hanging, it runs ``scancel`` on the jobstep id.

We need to generalize this to support job launchers other than SLURM. I think that most MPI job launchers will tear down an MPI job if one sends a signal like ``SIGINT`` or ``SIGKILL`` to the pid of the job launcher command (``mpirun``, ``jsrun``, etc). We should implement that method as the default and test it on SLURM ``srun``, LSF ``jsrun``, SLURM with MVAPICH2 ``mpirun_rsh``.

As a bonus, we could see whether it's possible to keep the existing SLURM-specific method as a specialization on the JobLauncher class if one used ``srun`` to launch a job.

Let's move the jobstep_id and kill_jobstep methods out of the resource manager class. Those should go in the job luancher class I think.

Start by locating the JobLauncher and resource manager implementations, focusing on jobstep_id and kill_jobstep. Compare the launcher-pid signal behavior with srun, jsrun, and mpirun_rsh; done means the default watchdog works across those launchers and the SLURM-specific behavior remains available for srun.

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