MDEV-23908: Implement SELECT ... OFFSET ... FETCH ...

This commit implements the standard SQL extension
OFFSET start { ROW | ROWS }
[FETCH { FIRST | NEXT } [ count ] { ROW | ROWS } { ONLY | WITH TIES }]

To achieve this a reserved keyword OFFSET is introduced.

The general logic for WITH TIES implies:
1. The number of rows a query returns is no longer known during optimize
   phase. Adjust optimizations to no longer consider this.
2. During end_send make use of an "order Cached_item"to compare if the
   ORDER BY columns changed. Keep returning rows until there is a
   change. This happens only after we reached the row limit.
3. Within end_send_group, the order by clause was eliminated. It is
   still possible to keep the optimization of using end_send_group for
   producing the final result set.

sql/structs.h

@@ -826,11 +826,13 @@ class Lex_select_limit
 public:
   /* explicit LIMIT clause was used */
   bool explicit_limit;
+  bool with_ties;
   Item *select_limit, *offset_limit;
 
   void clear()
   {
     explicit_limit= FALSE;    // No explicit limit given by user
+    with_ties= FALSE;         // No use of WITH TIES operator
     select_limit= NULL;       // denotes the default limit = HA_POS_ERROR
     offset_limit= NULL;       // denotes the default offset = 0
   }