Require explicit designation of proxy partitioning specification.
Require explicit designation of proxy partitioning specification.

Claiming to preserve Arbitrary by default was wrong and
downright dangerous. Turns out we were manually specifying
things everywhere, so I've made this a required argument.
(Required by keyword to avoid having to re-order arguments.)

apache/beam,lukecwik/incubator-beam,apache/beam,apache/beam,lukecwik/incubator-beam,chamikaramj/beam,lukecwik/incubator-beam,robertwb/incubator-beam,lukecwik/incubator-beam,apache/beam,apache/beam,lukecwik/incubator-beam,robertwb/incubator-beam,chamikaramj/beam,apache/beam,robertwb/incubator-beam,robertwb/incubator-beam,robertwb/incubator-beam,robertwb/incubator-beam,robertwb/incubator-beam,chamikaramj/beam,apache/beam,chamikaramj/beam,robertwb/incubator-beam,chamikaramj/beam,lukecwik/incubator-beam,chamikaramj/beam,chamikaramj/beam,apache/beam,apache/beam,robertwb/incubator-beam,chamikaramj/beam,chamikaramj/beam,lukecwik/incubator-beam,apache/beam,chamikaramj/beam,lukecwik/incubator-beam,lukecwik/incubator-beam,apache/beam,lukecwik/incubator-beam,robertwb/incubator-beam