suzerain: #3079 move to Allreduce logic

I am suspecting race-related issues of taking down at least
one if not two jobs on Lonestar.  By pushing the Allreduce
into the stack, instead of relying on a separate Reduce and
Bcast with intermediate computation, hopefully I'll pick
up some robustness here.

See discussion in the ticket.

git-svn-id: https://svn.ices.utexas.edu/repos/pecos/suzerain/trunk@44369 0bd85f0b-c151-43b9-a1e9-27231104551f