Goals

Fix GraphSyncing UnixFS for multiblock files based on test in #363

Implementation

update go-ipld-prime to master (need to tag)
update go-unixfsnode to master, then further apply fix contained in https://github.com/ipfs/go-unixfsnode/pull/28
modify the go-type for the ipld message v2 format, making extensions a map[string]*datamodel.Node instead of a map[string]datamodel.Node per @mvdan
modify the visitors on both sides of the request execution to read the entire file into ioutil.Discard, which triggers the loading of the files blocks. This causes the responder to send them over the wire and the requestor to verify them.
also, WOW -- did I not commit a bunch of changes to the docs? HA. Oh, right, I probably left them unsaved in VSCode.