This makes use of the new support in Djblets to request specific

fields/links in a payload. The rbt post command only fetches exactly

what's needed in every request it makes, as do some self-contained

utility functions. The end result is a small but measurable time savings

and roughly a 50-60% savings in total bytes transferred.

I've been using this change for local testing and for changes going into

reviews.reviewboard.org for the past week (for backwards-compatibility

testing). Haven't hit any issues yet.