@Override
  public int getLastPageIndex() {
    return lastPageIndex.get();
  }
when page prepares to insert, compare it with lastLeaf from context. if not same, then flush
the lastLeaf, unlock, deref, and remove it from dirtyPages. lastLeafPage only initiated at
overflowOperation
iotdb-core/datanode/src/main/java/org/apache/iotdb/db/schemaengine/schemaregion/mtree/impl/pbtree/schemafile/pagemgr/PageManager.java