My understanding is that an MPI_WAIT() on an inactive request should
return the empty status (MPI 3.0 page 52 line 46).
Make the TCP BTL really fail-safe. It now trigger the error callback on
all pending fragments when the destination goes down. This allows the PML
to recalibrate its behavior, either find an alternate route or just give up.
The patch from ticket #3469 adapted for the trunk.