Fix critical bug in session expiration after loading snapshot

This problem was introduced two days ago in commit 10b42c6ff. Session
modify times weren't being saved in snapshots, so after loading a
snapshot, a server would expire sessions too aggressively. This would
potentially cause state machines to diverge (resulting in consistency
problems).

The fix is straightforward: save the session modification time in the
snapshot along with the rest of the session fields.