MDEV-34720: Poor plan choice for large JOIN with ORDER BY and small LIMIT

(Variant 2b: call greedy_search() twice, correct handling for limited
 search_depth)

Modify the join optimizer to specifically try to produce join orders that
can short-cut their execution for ORDER BY..LIMIT clause.

The optimization is controlled by @@optimizer_join_limit_pref_ratio.
Default value 0 means don't construct short-cutting join orders.
Other value means construct short-cutting join order, and prefer it only
if it promises speedup of more than #value times.

In Optimizer Trace, look for these names:
* join_limit_shortcut_is_applicable
* join_limit_shortcut_plan_search
* join_limit_shortcut_choice

sql/sql_select.h

@@ -1205,6 +1205,20 @@ public:
     passing 1st non-const table to filesort(). NULL means no such table exists.
   */
   TABLE    *sort_by_table;
+
+  /*
+    If true, there is ORDER BY x LIMIT n clause and for certain join orders, it
+    is possible to short-cut the join execution, i.e. stop it as soon as n
+    output rows were produced. See join_limit_shortcut_is_applicable().
+  */
+  bool    limit_shortcut_applicable;
+
+  /*
+    Used during join optimization: if true, we're building a join order that
+    will short-cut join execution as soon as #LIMIT rows are produced.
+  */
+  bool    limit_optimization_mode;
+
   /* 
     Number of tables in the join. 
     (In MySQL, it is named 'tables' and is also the number of elements in