Returns Future(true) to indicate that the process is alive but does not mean that it is ready to receive traffic e.g.
Returns Future(true) to indicate that the process is alive but does not mean that it is ready to receive traffic e.g. is has not joined the cluster or is loading initial state from a database
Returns Future(result) containing the system's liveness result
Returns Future(true) if the system is ready to receive user traffic
Returns Future(result) containing the system's readiness result
Not for user extension