ENH: use bundled MPI_Test when checking processor interface ready()

- fewer calls, potentially more consistent

ENH: update sendRequest state after recvRequest wait

- previously had this type of code:

    // Treat send as finished when recv is done
    UPstream::waitRequest(recvRequest_);
    recvRequest_ = -1;
    sendRequest_ = -1;

  Now refined as follows:

    // Require receive data. Update the send request state.
    UPstream::waitRequest(recvRequest_);
    recvRequest_ = -1;
    if (UPstream::finishedRequest(sendRequest_)) sendRequest_ = -1;

  Can potentially investigate with requiring both,
  but this may be over-contrained.
  Example,

    // Require receive data, but also wait for sends too
    UPstream::waitRequestPair(recvRequest_, sendRequest_);