Second stage of optimizer_trace optimizations

- Move testing of my_writer to inline functions to avoid calls
- Made more functions inline. Especially thd->thread_started()
  is now very optimized!
- Moved Opt_trace_stmt classe to opt_trace_context.h to get critical
  functions inline

sql/my_json_writer.cc

@@ -260,50 +260,6 @@ void Json_writer::add_str(const String &str)
   add_str(str.ptr(), str.length());
 }
 
-Json_writer_object::Json_writer_object(THD *thd) :
-  Json_writer_struct(thd)
-{
-  if (unlikely(my_writer))
-    my_writer->start_object();
-}
-
-Json_writer_object::Json_writer_object(THD* thd, const char *str) :
-  Json_writer_struct(thd)
-{
-  if (unlikely(my_writer))
-    my_writer->add_member(str).start_object();
-}
-
-Json_writer_object::~Json_writer_object()
-{
-  if (my_writer && !closed)
-    my_writer->end_object();
-  closed= TRUE;
-}
-
-Json_writer_array::Json_writer_array(THD *thd) :
-  Json_writer_struct(thd)
-{
-  if (unlikely(my_writer))
-    my_writer->start_array();
-}
-
-Json_writer_array::Json_writer_array(THD *thd, const char *str) :
-  Json_writer_struct(thd)
-{
-  if (unlikely(my_writer))
-    my_writer->add_member(str).start_array();
-
-}
-Json_writer_array::~Json_writer_array()
-{
-  if (unlikely(my_writer && !closed))
-  {
-    my_writer->end_array();
-    closed= TRUE;
-  }
-}
-
 Json_writer_temp_disable::Json_writer_temp_disable(THD *thd_arg)
 {
   thd= thd_arg;