Group-based expiration

Now that WildFly represents a web session as a group of cache entries (instead of a single entry containing an atomic map), there are a few hurdles preventing us from leveraging infinispan-managed expiration.
One of which is the fact that expiration of cache entries within a group must be atomic. It would be very bad if a request arrives for a session whose entries are in the process of being expired. The expiration thread should obtain a lock on all the keys for a group (or at least some pre-determined "primary" key), so that late session access doesn't result in session attributes mysteriously disappearing (because they were independently expired). This can also cause integrity constraint violations if cache entries reference other entries in the group. e.g. currently the "primary" cache entry for a session (keyed by session id), contains a set of attribute names. These names correspond to other cache entries that contain the value of a given attribute for that session.