MVStore: use a mark & sweep GC algorithm instead of reference counting, to…

MVStore: use a mark & sweep GC algorithm instead of reference counting, to ensure used chunks are never overwrite, even if the reference counting algorithm does not work properly.

h2/src/test/org/h2/test/store/TestConcurrent.java

@@ -285,6 +285,8 @@ public class TestConcurrent extends TestMVStore {
             MVMap<Integer, Integer> m = s.openMap("dummy");
             m.put(1, 1);
             s.commit();
+            m.put(2, 2);
+            s.commit();
             
             MVMap<String, String> meta = s.getMetaMap();
             int chunkCount = 0;