[CP] Make estimate memtable row stable && fix mysql test cases

src/storage/memtable/mvcc/ob_keybtree.cpp

@@ -1663,6 +1663,7 @@ int ObKeyBtree<BtreeKey, BtreeVal>::destroy(const bool is_batch_destroy)
     WaitQuiescent(get_qsync());
     tg.click();
   }
+  split_info_ = 0;
   return OB_SUCCESS;
 }