[MPICH] IMB Exchange

[Calin Iaru]

I am looking at IMB Excahnge implementation which is:
  - asynchronous send to left and right rank
  - blocking receive from left and right
  - Waitall
What I don't understand is why is blocking receive followed by Waitall. 
If all nodes are doing async send which are not guaranteed to complete, 
then the receive operation is not satisfied. Let me know your opinion. I 
would expect that waitall is after async send.