public class InSakaiSessionTSH
A TokenStateHolder that stores flow state in the Sakai Tool-specific session,
falling back to the global Sakai Session if no ToolSession is active (in the
case we are operating without a tool placement for some reason).
NB Expiryseconds not yet implemented. Would require *extra* server-side
storage of map of tokens to sessions, in order to save long-term storage
within sessions - awaiting research from performance clients.