404 responses should re-resolve the parent

[ssfrr]

We changed some of the URL structure in ChainAPI, and the plan was to do 301 redirects so clients holding old URLs would still work fine, but it turns out redirects and POSTs don't play nicely, so clients with cached POST URLs were SOL.

ChainClient could automatically keep a reference to where it got each link from, and then if it ever gets a 404 it could re-request that object (and recursively do the same thing if THAT request gave a 404). That would be transparent from the application side.