sewer ignores fresh nonces in most responses, makes get-nonce requests too often

[mmaney]

What is it that you would like to propose to add/remove/change?

Add a trivial cache to save the fresh nonces the server returns in most responses and use them when available instead of waiting for a new-nonce query.

Why do you want to add/remove/change that?

Politeness (stop squandering nonces that the server has to track), efficiency (the roundtrip to get a nonce for almost every request isn't huge, but this change saved 7 out of 9 new-nonce queries in my simple one-cert test).

How do you want to go about adding/removing/changing that?

Review PR to be attached here. NB: this builds on top of the wrap-requests PR, so if the latter isn't merged this one will... actually, I guess it would pull wrap-requests in, wouldn't it? If not, it would fail and have to be recreated, basically, with the caching done from multiple places.