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/handler.h

@@ -3557,6 +3557,11 @@ public:
     if (org_keyread != MAX_KEY)
       ha_start_keyread(org_keyread);
   }
+
+protected:
+  bool is_root_handler() const;
+
+public:
   int check_collation_compatibility();
   int check_long_hash_compatibility() const;
   int ha_check_for_upgrade(HA_CHECK_OPT *check_opt);