add recursion argument to various platform agent commands

- SHUTDOWN command added
- 'recursion' parameter added to INITIALIZE RESET GO_ACTIVE GO_INACTIVE RUN SHUTDOWN
- improved consistency of method named related with operations involving children
- _terminate_subplatform_agent_processes and _terminate_instrument_agent_processes now only called in _shutdown
- pass log=sys.stdout argument to WSGIServer to avoid/reduce the lots of logging lines printed to the default stderr and that appear in the output in some of the buildbots even when -s is not used
- include SHUTDOWN in all platform integration tests, so the various child processes get terminate properly (and avoiding leaks)