MDEV-406: ANALYZE $stmt: get ANALYZE work for subqueries

- "ANALYZE $stmt" should discard select's output, but it should still
  evaluate the output columns (otherwise, subqueries in select list
  are not executed)
- SHOW EXPLAIN's code practice of calling JOIN::save_explain_data()
  after JOIN::exec() is disastrous for ANALYZE, because it resets
  all counters after the first execution. It is stopped
  = "Late" test_if_skip_sort_order() calls explicitly update their part
    of the query plan.
  = Also, I had to rewrite I_S optimization to actually have optimization
    and execution stages.

sql/sql_parse.cc

@@ -5260,10 +5260,13 @@ static bool execute_sqlcom_select(THD *thd, TABLE_LIST *all_tables)
     {
       //psergey-todo: ANALYZE should hook in here...
       select_result *save_result;
+      Protocol *save_protocol;
       if (lex->analyze_stmt)
       {
         save_result= result;
         result= new select_send_analyze();
+        save_protocol= thd->protocol;
+        thd->protocol= new Protocol_discard(thd);
       }
       else
       {
@@ -5280,6 +5283,7 @@ static bool execute_sqlcom_select(THD *thd, TABLE_LIST *all_tables)
         result= save_result;
         if (!result && !(result= new select_send()))
           return 1;
+        thd->protocol= save_protocol;
         thd->lex->explain->send_explain(thd);
 
         if (result != lex->result)