Patch from Charlie to update to Google Collect rc1.

src/java/com/samskivert/depot/EHCacheAdapter.java

@@ -24,6 +24,7 @@ import java.io.Serializable;
 import java.util.Collections;
 import java.util.Map;
 import java.util.Set;
+import java.util.concurrent.ConcurrentHashMap;
 
 import com.google.common.collect.Maps;
 import com.google.common.collect.Sets;
@@ -277,7 +278,8 @@ public class EHCacheAdapter
         protected Ehcache _cache;
         protected String _id;
 
-        protected Set<Serializable> _keys = Sets.newConcurrentHashSet();
+        protected Set<Serializable> _keys =
+            Sets.newSetFromMap(new ConcurrentHashMap<Serializable, Boolean>());
     }
 
     // this is just for convenience and memory use; we don't rely on pointer equality anywhere