#4 MPI_Waitsome/Waitany, MPI_ANY_SOURCE

[David Skinner]

Currently the recovery of buffersizes from MPI_Waitsome
and Waitany is lacking. One could compute mean
buffersizes in order to get at least a statistical
measure of buffer sizes.

Likewise there is one bit left in the first 64 bit
keys. One could use this key to register a rank that
does not make sense or is unknown. We can't use "0"
since that's a valid rank and "-1" doesn't fit into the
bitstructure.

-David