MDEV 7701 extra() calls for VP engine

Added Spider patches:
003_mariadb-10.0.15.vp.diff
060_mariadb-10.2.0.partition_reset_top_table_fields.diff

- Support HA_EXTRA_ADD_CHILDREN_LIST,HA_EXTRA_ATTACH_CHILDREN,
  HA_EXTRA_IS_ATTACHED_CHILDREN and HA_EXTRA_DETACH_CHILDREN
  in partition handler for handlers that has HA_CAN_MULTISTEPL_MERGE flag
- Added HA_CAN_MULTISTEPL_MERGE to MERGE handler.
- Added handler::get_child_handlers()
- Change m_num_lock to contain total number of locks. This was needed as
  we now adjust number of locks when extra(HA_EXTRA_ATTACH_CHILDREN) is
  called.

sql/handler.cc

@@ -6000,6 +6000,7 @@ int handler::ha_reset()
   /* Reset information about pushed engine conditions */
   cancel_pushed_idx_cond();
   /* Reset information about pushed index conditions */
+  clear_top_table_fields();
   DBUG_RETURN(reset());
 }