performance optimization: Upgrade Hibernate to 3.6.8 version when it is released

Upgrade Hibernate to 3.6.8 version when it is released.

There is a performance problem when HQL queries are cached. It is being fixed:
https://hibernate.onjira.com/browse/HHH-5927

org.hibernate.util.SoftLimitMRUCache.get(Object) (called by org.hibernate.engine.query.QueryPlanCache.getHQLQueryPlan(String, boolean, Map)) is showing up as the top blocker in a performance test of a Grails application.

There is already a patch available:
https://github.com/stliu/hibernate-core/tree/HHH-5297-36

Notice:
This patch adds a dependency to Google Guava 10.0 .
If this is the fix that gets accepted to Hibernate, "runtime 'com.google.guava:guava:10.0'" dependency should be added to Grails dependencies. Consider removing the dependency to com.googlecode.concurrentlinkedhashmap and replace that LRU cache by Google Guava's implementation.