MDEV-31033 ER_KEY_NOT_FOUND upon online COPY ALTER on a partitioned table

The row events were applied "twice": once for the ha_partition, and one
more time for the underlying storage engine.

There's no such problem in binlog/rpl, because ha_partiton::row_logging
is normally set to false.

The fix makes the events replicate only when the handler is a root handler.
We will try to *guess* this by comparing it to table->file. The same
approach is used in the MDEV-21540 fix, 231feabd. The assumption is made,
that the row methods are only called for table->file (and never for a
cloned handler), hence the assertions are added in ha_innobase and
ha_myisam to make sure that this is true at least for those engines

Also closes MDEV-31040, however the test is not included, since we have no
convenient way to construct a deterministic version.

sql/sql_class.h

@@ -7561,6 +7561,22 @@ inline bool handler::has_long_unique()
   return table->s->long_unique_table;
 }
 
+/**
+  Return whether the handler is root.
+  @return false if table is maintained by different handlerton, true otherwise.
+  @note The implementation supposes that the same handler can't be found as both
+  root and non-root.
+
+  There are two known cases when it's non-root:
+  1. under partition's ha_write_row() (also true for copy_partitions())
+  2. under ha_mroonga::wrapper_write_row();
+  same applies for ha_delete_row/ha_update_row.
+*/
+inline bool handler::is_root_handler() const
+{
+  return ht == table->file->ht;
+}
+
 extern pthread_attr_t *get_connection_attrib(void);
 
 /**