Auth subsystem (rebased)

[s-kostyuk]

Implements the new Authentication and Authorization subsystem.

Breaking changes:

• removed an old auth subsystem altogether;
• ApiGateway now uses AuthService instead of AuthManager for authentication and authorization;
• changed a return type of generate_token method from the dpl.utils package (bdb6ec1).

Enhancements:

• the 'Session' term was introduced;
• Session represents a session of communication between an everpl instance and a client application;
• each Session has an attached access token (key) which is used by client devices for identification and authorization in the system;
• declared SessionRepository and SessionService;
• implemented SessionRepository for in-memory storage of Sessions;
• implemented SessionService;
• declared a new AuthService;
• implemented AuthService on-top of SesionService and UserService from #57;
• prepared a basis for permission checking logic (5f24dd5).

Closes #39