MVStore: support concurrent transactions (MVCC style, should match the behavior of PostgreSQL)

h2/src/main/org/h2/mvstore/MVMap.java

@@ -96,6 +96,7 @@ public class MVMap<K, V> extends AbstractMap<K, V>
      */
     @SuppressWarnings("unchecked")
     public V put(K key, V value) {
+        DataUtils.checkArgument(value != null, "The value may not be null");
         beforeWrite();
         try {
             long writeVersion = store.getCurrentVersion();