module provides session variables protected by authorization token.
Session token represents tuple cached in ETS.
It consists of session id, key, expiration time and value.
Session token used both for auth and for session variables.
Session token encoded with pickler tagged as 'Token' tuple.
The protocol is following. If session is expired or session
token is invalid then new session token issued. If session token
is not expired and valid then two cases could happen:
1) if nitro_prolongate n2o parameter is true,
then new expiration value is updated for current session, while
session sid remains unchanged;
2) if nitro_prolongate is false then full session token remains unchanged.