[CommCoreModule] Fill out GRPCStreamHostObject:close() constructor

Summary: GRPCStream:close() -> GlobalNetworkSingleton.scheduleOrRun(.. -> NetworkModule.closeGetStream() -> Client.closeGetStream() -> ClientGetReadReactor.context.TryCancel()

Test Plan:

React.useEffect(() => {
  const callbackFn = arg => {
    console.log(`The payload is ${JSON.stringify(arg)}`);
  };

  const blah: GRPCStream = global.CommCoreModule.openSocket();
  blah.onmessage = callbackFn;

  blah.close();
}, []);

Reviewers: ashoat, palys-swm, def-au1t, karol-bisztyga

Reviewed By: ashoat, palys-swm, karol-bisztyga

Subscribers: Adrian, benschac

Differential Revision: https://phabricator.ashoat.com/D3052