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.cc

@@ -2205,11 +2205,6 @@ void THD::reset_globals()
   net.thd= 0;
 }
 
-bool THD::trace_started()
-{
-  return opt_trace.is_started();
-}
-
 /*
   Cleanup after query.