Code review comment for lp:~djfroofy/txaws/newagent-767205

This change looks good to me at first glance (reading the diff on the merge proposal; I haven't branched the code yet). In particular, the amz_headers functionality is much-needed.

Drew, I don't see anything about the new web client usage, only headers and body producer. Is that coming in a future change?

At the risk of being a total pain in the ass, here's what I would recommend:

1) split out the headers changes into a new branch, and attach it to bug #972432.
2) create a new bug for the body producer/receiver factory feature, split that code out, and attach it to there in its own branch.
3) do the work on the new agent in this branch and this ticket (bug #767205).