Work around IDEA-126899
Remove Serializable from Session

Fixes #15
Remove Sesion.setLastAccessedTime

Fixes #16
Move session.web to session.web.http

Fixes #18
Create ExpiringSession which extends Session

This provides a better separation for consumers of the API. Most users are
likely not interested in checking to see if a session is expired so they
can focus on the Session API.

Fixes #28
Reorder SessionRepository methods

Fixes #29