In previous implementation PToken contained the full Token structure.
Since private token is used for data signature only, storing unused
fields of a user token is impractical. To emphasize the purpose of
the private part of the session, it makes sense to provide the user
of the session package with its interface. The interface will only provide
the functionality of data signing with private session key.
This commit:
* removes PToken structure from session package;
* defines PrivateToken interface of private session part;
* adds the implementation of PrivateToken on unexported struct;
* provides the constructor that generates session key internally.
