Default LibclusterMesh.Runner, running the mesh CLI through a port.
A missing binary, a non-zero exit and a daemon that never answers all come
back as {:error, message} rather than raising, and timeout bounds the
whole call, so one poll is the most any of them can cost.
A command still running at timeout is signalled by pid. Closing a port
frees the file descriptors and leaves the program behind it running, and a
status command blocked on a wedged daemon socket has nothing to notice, so
merely abandoning it would leave one more of them behind on every poll for
as long as the daemon stays wedged.