graphsync spec - clarify relationships between libp2p connections, streams, messages and requests about specs HOT 1 OPEN

Notes from discussion with @hannahhoward
*) go-graphsync design was based on bitswap implementation which sends blocks on a separate stream
*) seems reasonable to be able to allow responses back over incoming stream - if we do this, go-graphsync should be updated to do this as well.
*) go-graphsync will in some case send messages with request, response and data
*) metadata extension provides mapping between responses and blocks!! this should be moved into the core protocol!!
*) metadata extension also specifies whether or not responder has block!! - in this case, requester should not fail selector traversal validation...
*) requester will silently ignore/discard blocks not referenced in the metadata!!

from specs.