End-to-end support for new HTTP storage protocol: tracking issue

[itamarst]

Each of these will be zero or more additional tickets:

1. #3902: Storage nodes can be started with HTTP protocol (It is maybe possible but not worth doing to do this via storage plugin.)
2. Storage nodes send the NURL to an introducer, in addition to the fURL for legacy Foolscap protocol. This is sent as an announcement upgrade so it supercedes existing announcement.
3. The introducer sends NURLs to storage clients. INVARIANT: old storage clients can safely get and ignore NURLs. If this is not the case, some conditional logic needs to be introduced so old clients don't get NURLs.
4. The storage client understands NURLs, and use HTTP protocol instead of Foolscap when possible. See design doc for further discussion (QUESTION: is it actually possible to say "this furl and nurl are for the same server"? technically, maybe but not necessary given announcement upgrades).
5. Currently the storage client caches per-server FURLs indefinitely. New clients should check with introducer if a NURL (HTTP) has replaced a FURL (Foolscap) and update appropriately.
6. The client may only have FURL, and not have introducer. For that situation the Foolscap protocol should grow a away for clients to get new URLs (remote_get_version?)
7. Update the documentation appropriately.