AMPI #1084: eliminate extra copies in reduction/gather recv paths
The old control flow for reduction recv's was: reduceResult -> sendraw
-> generic -> inorder -> receive.
The new control flow is rednResult -> receive. This way we do not copy
from CkReductionMsg to AmpiMsg.
Gathers have the same control flow as reductions, but the request is
different in order to process the message appropriately.