seatbelt: add a fallback handler that acts on input, not output

[afoxman]

In seatbelt, all of the fallback handlers act on the output. I have a scenario in which my fallback behavior is to re-issue the failing request to a different endpoint. To do that, I need a clone of the original request.

At the moment, I am having to work around this with some ugly acrobatics in my service (copy each request to each output, for use in fallback).

Ideally, the request cloning would be limited to cases where an input-centric fallback action is in play. Output-centric fallback actions wouldn't incur that overhead.

[afoxman]

Turns out what I want here is the Retry layer, so closing this in lieu of that existing and doing what I need. Nice bonus - using Retry means when I re-run another request, it feeds back into the service layers.