ENH: align MPI_Probe non-blocking handling with header description
- the header states that the commsType is non-blocking or not,
but the implementation actually checked for 'buffered' or not.
STYLE: fix some spacing and some documentation
- this simplifies polling receives and allows separation from
the sends
ENH: add UPstream::removeRequests(pos, len)
- cancel/free of outstanding requests and remove segment from the
internal list of outstanding requests