optimize array-of-request completions

[jeffhammond]

cleanup_mapped_request does a check on .empty() for all the relevant maps every call. We can hoist this to optimize the common case.

    for (int i=0; i<count; i++) {
        if (!ignore) {
            MPI_Status_to_WRAP_Status(&impl_statuses[i], &array_of_statuses[i]);
        }
        if (impl_requests[i] == MPI_REQUEST_NULL) {
            cleanup_mapped_request(CONVERT_MPI_Request(array_of_requests[i]));
        }
        array_of_requests[i] = OUTPUT_MPI_Request(impl_requests[i]);
    }