MDEV-16978 Application-time periods: WITHOUT OVERLAPS

* The overlaps check is implemented on a handler level per row command.
  It creates a separate cursor (actually, another handler instance) and
  caches it inside the original handler, when ha_update_row or
  ha_insert_row is issued. Cursor closes on unlocking the handler.

* Containing the same key in index means unique constraint violation
  even in usual terms. So we fetch left and right neighbours and check
  that they have same key prefix, excluding from the key only the period part.
  If it doesnt match, then there's no such neighbour, and the check passes.
  Otherwise, we check if this neighbour intersects with the considered key.

* The check does not introduce new error and fails with ER_DUPP_KEY error.
  This might break REPLACE workflow and should be fixed separately

sql/sql_class.cc

@@ -175,7 +175,8 @@ Key::Key(const Key &rhs, MEM_ROOT *mem_root)
   columns(rhs.columns, mem_root),
   name(rhs.name),
   option_list(rhs.option_list),
-  generated(rhs.generated), invisible(false)
+  generated(rhs.generated), invisible(false),
+  without_overlaps(rhs.without_overlaps), period(rhs.period)
 {
   list_copy_and_replace_each_value(columns, mem_root);
 }