fix(chainbase): make Snapshot head field volatile in Chainbase

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: Little-Peony

chainbase/src/main/java/org/tron/core/db2/core/Chainbase.java

@@ -35,7 +35,7 @@ public enum Cursor {
   //true:fullnode, false:soliditynode
   private ThreadLocal<Cursor> cursor = new ThreadLocal<>();
   private ThreadLocal<Long> offset = new ThreadLocal<>();
-  private Snapshot head;
+  private volatile Snapshot head;
 
   public Chainbase(Snapshot head) {
     this.head = head;