PID of commands run with `CommandProtocol` are now tracked in memory and stopped/killed on
exit. A `pid_file` argument can be use to track the file on disk, which is useful to
detect stalled processes after a crash/unclean ending of the backend.
Add helper methods to stop and kill those processes.
`init_script_path` option can be used in `[DEFAULTS]` to run a script at the end of
backend initialisation. A new `init_pre_script` method is used to wait for backend to
reach this stage (designed to be used mostly by CLI frontend), then the usual `ready_get`
method is finished once the script is finished.
