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

[itamarst]

Each of these will be zero or more additional tickets:

1. Storage nodes can be started with HTTP protocol (QUESTION: is it worth doing this via storage plugin?)
2. Storage nodes send the NURL to an introducer, in addition to the fURL for legacy Foolscap protocol.
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"?).
5. Currently the storage client caches per-server FURLs indefinitely. New clients should check if a NURL (HTTP) has replaced a FURL (Foolscap) and update appropriately.