Reworked Session to make the expiration a store's responsibility and

Reworked Session to make the expiration a store's responsibility and
fix the bug in state where it handles expires as an object state.

psgi.streaming is supported in most Plack servers especially
Standalone, so there's no need to depend on AnyEvent in the test.

Added $session->commit now that all write code to the database is
delayed till the finalization phase.