Graph store protocol editor's draft updated

Updates to GSP to address WG consensus regarding:
 - RFC 2119 language in informative sections
 - Removed Protocol service discovery section 5.8 (addressing issue of
confusion regarding SPARQL protocol URL and that of a GSP
implementation)
 - Changed URL used to for indirect identification to reflect that it
identifies a graph store (removed all references to 'service')

-- Chime