Server shutdown RPC service

[scholarsmate]

There needs to be an RPC service that can shutdown the RPC server.

The rpc service can be defined as follows:

service Editor {
  ...
  rpc ServerControl(ServerControlRequest) returns (ServerControlResponse);
}

enum ServerControlKind {
  SERVER_CONTROL_UNDEFINED = 0;
  SERVER_CONTROL_GRACEFUL_SHUTDOWN = 1; // server will stop accepting new sessions and will exit when all sessions are destroyed
  SERVER_CONTROL_IMMEDIATE_SHUTDOWN = 2; // server will stop accepting new sessions and will exit immediately
}

message ServerControlRequest {
  ServerControlKind kind = 1; // server control kind
}

message ServerControlResponse {
  ServerControlKind kind = 1; // server control kind
  int32 response_code = 2; // response code, 0 for success, non-zero for failure
}