Use State.get to query whether the current process has been initialized as an rpc
parallel master (start_app or init_master_exn has been called). We return a
State.t rather than a bool so that you can require evidence at the type level.
If you want to certify, as a precondition, for some function that start_app was
used, require a State.t as an argument. If you don't need the State.t anymore,
just pattern match on it.