Session expiration handler

This pull request stems from two related issues …

**[IMPORTANT] Adding session expiration to the DatabaseSessionHandler**

By default each request has a 2% chance of running the garbage collector. The database is also agnostic of any expiration, so if a session ID is spoofed in a cookie, and the garbage collector has not been run, the session can be hijacked regardless of whether the session has expired or not. This means that the majority of the onus is put on the cookie to expire in order to check the session validity.

This fix addresses this issue by ensuring the session is still valid before it is used, even if the garbage collector has not been run.

**Abstracting session expiration**

The session expiration has repeated code amongst many of the handlers, so this introduces the interface ExpirationAwareInterface which contains three method: *getLifetime*, *setLifetime* and *garbageCollect*.

The methods getLifetime and setLifetime provide accessors and mutators in the same workings as the interface ExistenceAwareInterface. It provides a universal way to passthrough the term in which a session is expected to exist in its handling endpoint.

The garbageCollect method is a simple courtesy, it calls the gc method (part of PHP's SessionHandlerInterface interface) with the maxLifetime parameter pre-filled.