Merge 10.4 into 10.5

A conflict between MDEV-19514 (b42294bc64)
and MDEV-20934 (d7a2401750)
was resolved. We will not invoke the function ibuf_delete_recs()
from ibuf_merge_or_delete_for_page(). Instead, we will add that
logic to the function ibuf_read_merge_pages().

sql/sql_select.cc

@@ -9831,7 +9831,10 @@ prev_record_reads(const POSITION *positions, uint idx, table_map found_ref)
           #max_nested_outer_joins=64-1) will not make it any more precise.
       */
       if (pos->records_read)
+      {
         found= COST_MULT(found, pos->records_read);
+        found*= pos->cond_selectivity;
+      }
      }
   }
   return found;