- broadcasts list contiguous content as a two-step process:
    1. broadcast the size, and resize for receiver list
    2. broadcast contiguous contents (if non-empty)

  This avoids serialization/de-serialization memory overhead but at
  the expense of an additional broadcast call.
  The trade-off of the extra broadcast of the size will be less
  important than avoiding a memory peak for large contiguous mesh data.

REVERT: unstable MPI_Mprobe/MPI_Mrecv on intelmpi + PMI-2 (#2796)

- partial revert of commit c6f52858, for NBX implementation.
  Not yet flagged as causing errors here, but eliminated for
  consistency.