[Mpi3-bwcompat] MPI Timer feedback

[Fab Tillier]

Feedback on MPI Timer requests presentation:
- MPI_Request_free is ambiguous in the standard.  We should define specific requirement that MPI_Request_free actually frees timer requests, rather than leak or provide some other indeterminate behavior.
- MPI_Timer_reset should take request by value, not ref, as it doesn't free the request handle.
- Some folks suggested they would rather see new 'poll' functions that take a timeout as a parameter rather than as a timer request.  This potentially avoids having to scan input requests looking for minimum time.  However, it requires special handling for reporting timeouts.
- Need text to prevent implementations ignoring MPI_Cancel.  Text in standard about local operations must complete, leverage that.