Javadoc
A session has been requested by its id on this node.
Load the session by id AND context path from the database.
Multiple contexts may share the same session id (due to dispatching)
but they CANNOT share the same contents.
Check if last node id is my node id, if so, then the session we have
in memory cannot be stale. If another node used the session last, then
we need to refresh from the db.
NOTE: this method will go to the database, so if you only want to check
for the existence of a Session in memory, use _sessions.get(id) instead.