New implementation for A2A: use mpi_allgather and mpi_reduce_scatter,
with two variants: one for variable inputs, one for fixed size
inputs.
Modified graph_fnd_owner to pass padded and uniform sized input to
A2A.
Reimplement adjcncy_fnd_owner to use alltoallv. Version with
sends/irecv still available under compile time constant.
Reimplement a2a_fnd_owner to use adjcncy_fnd_owner. Older version
still available under compile time constant.