model change: StorageSite

[pdowler]

StorageSite could expose the signature of the public key(s) available; then raven could check that a site will be able to validate a pre-auth url before generating one.

StorageSite -> 0,* PubKeySig
0: minoc doesn't have a pub key
*: multiple pub keys from multiple transfer negotiation services (raven, vospace, ...)

This would eliminate the scenario where raven generates URLs that fail because the minoc instance doesn't have the pub key.