Improved speed of optimizer trace

- Added unlikely() to optimize for not having optimizer trace enabled
- Made THD::trace_started() inline
- Added 'if (trace_enabled())' around some potentially expensive code
  (not many found)
- Added ASSERT's to ensure we don't call expensive optimizer trace calls
  if optimizer trace is not enabled
- Added length to Json_writer functions to speed up buffer writes
  when optimizer trace is enabled.
- Changed LEX_CSTRING argument handling to not send full struct to writer
  function on_add_str() functions now trusts length arguments

sql/sql_class.h

@@ -3274,7 +3274,10 @@ public:
   void reset_for_reuse();
   bool store_globals();
   void reset_globals();
-  bool trace_started();
+  bool trace_started()
+  {
+    return opt_trace.is_started();
+  }
 #ifdef SIGNAL_WITH_VIO_CLOSE
   inline void set_active_vio(Vio* vio)
   {
@@ -5003,6 +5006,7 @@ public:
   Item *sp_fix_func_item(Item **it_addr);
   Item *sp_prepare_func_item(Item **it_addr, uint cols= 1);
   bool sp_eval_expr(Field *result_field, Item **expr_item_ptr);
+
 };
 
 /** A short cut for thd->get_stmt_da()->set_ok_status(). */