Removes most instances of pb.SignedMessage (there's more to take out but they shouldn't hurt anyone as is).
There used to be places in psserver where a PieceID was hmac'd with the SatelliteID, which was gotten from a SignedMessage. This PR makes it so some functions access the SatelliteID from the Payer Bandwidth Allocation instead.
This requires passing a SatelliteID into psserver functions where they weren't before, so the following proto messages have been changed:
* PieceId - satellite_id field added
This is so the psserver.Piece function has access to the SatelliteID when it needs to get the namespaced pieceID.
This proto message should probably be renamed to PieceRequest, or a new PieceRequest message should be created so this isn't misnamed.
* PieceDelete - satellite_id field added
This is so the psserver.Delete function has access to the SatelliteID when receiving a request to Delete.
this change removes the cryptopasta dependency.
a couple possible sources of problem with this change:
* the encoding used for ECDSA signatures on SignedMessage has changed.
the encoding employed by cryptopasta was workable, but not the same
as the encoding used for such signatures in the rest of the world
(most particularly, on ECDSA signatures in X.509 certificates). I
think we'll be best served by using one ECDSA signature encoding from
here on, but if we need to use the old encoding for backwards
compatibility with existing nodes, that can be arranged.
* since there's already a breaking change in SignedMessage, I changed
it to send and receive public keys in raw PKIX format, instead of
PEM. PEM just adds unhelpful overhead for this case.
