Updated client awareness of session expiration

Changed client code to read new Protobuf fields for session max
length and grace period. Changed database handler to store extra
session information and check for session expiration.
If a session is expired - either because it has exceeded its max
lifespan, or because it has exceeded its grace period for renewal
- the client will discard the session information.
Co-commits:
svmp-protocol-def
svmp-server