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.
