ENH: MPI_Waitany / MPI_Waitsome interface for internal list of requests

- checks requests from completion, returning true when some requests
  have completed and false when there are no active requests.
  This allows it to be used in a polling loop to progress MPI
  and then respond when as requests become satisfied.

  When using as part of a dispatch loop, waitSomeRequests() is
  probably more efficient than calling waitAnyRequest() and can help
  avoid biasing which client requests are serviced.

  Takes an optional return parameter, to retrieve the indices,
  but more importantly to avoid inner-loop reallocations.

  Example,

      DynamicList<int> indices;
      while (UPstream::waitSomeRequests(startRequest, &indices))
      {
          // Dispatch something ....
      }

      // Reset list of outstanding requests with 'Waitall' for safety
      UPstream::waitRequests(startRequest);

  ---

  If only dealing with single items and an index is required for
  dispatching, it can be better to use a list of UPstream::Request
  instead.

  Example,

      List<UPstream::Request> requests = ...;

      label index = -1;
      while ((index = UPstream::waitAnyRequest(requests)) >= 0)
      {
          // Do something at index
      }

ENH: pair-wise wrappers for MPI_Test or MPI_Wait

- for send/recv pairs of requests, can bundle both together and use a
  single MPI_Testsome and MPI_Waitall instead of two individual
  calls.