prop140: Clients request diffs and handle diffs in replies

For the final piece of prop140, clients should ask for consensus diffs as appropriate, and handle them if they're received.

We may need proposal extensions here:

Should clients begin doing this immediately, or should there be a tristate where "default" means "look at the consensus"?
Should there be an option -- maybe for testing, maybe not -- that forces clients to look for directory guards that support consensus diffs?

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