Liveness watchdog for the supervision stream. If no
supervision message (healthy or unhealthy) is observed within
this duration, the controller is assumed to be unreachable and
the mesh is treated as unhealthy. This timeout is about
detecting silence, not slow messages.
This value must be > poll frequency + get actor state timeout + get proc state timeout
or else it is possible to declare the controller dead before it could
feasibly have received a healthy reply.