@Override
        public ICachedMNode next() {
          return children.poll();
        }
Since transplant and replacement may invalidate page instance held by a blocked read-thread,
the read-thread need to validate its page object once it obtained the lock.

@param initPage page instance with read lock held
@param parent MTree node to read children, containing latest segment address
@param cxt update context if page/segment modified
@return validated page
iotdb-core/datanode/src/main/java/org/apache/iotdb/db/schemaengine/schemaregion/mtree/impl/pbtree/schemafile/pagemgr/BTreePageManager.java