Code cleanup (#1534)

* removed unused imports (no code change)

* Add missing @Override / @SupressWarnings (no code change)

* FIX: serialization warnings

* FIX: Access to static member

src/main/java/io/ebeaninternal/server/cache/DefaultServerQueryCache.java

@@ -19,6 +19,7 @@ public class DefaultServerQueryCache extends DefaultServerCache {
     this.queryCacheEntryValidate = config.getQueryCacheEntryValidate();
   }
 
+  @Override
   protected Object unwrapEntry(CacheEntry entry) {
     return ((QueryCacheEntry) entry.getValue()).getValue();
   }