Function on_php_script_finish_ok

this function is called after shutdown functions have been executed (probably, after timeout) what we do here: 1) invoke php callback — it returns whether we should flush this trace or not (we are in script context, so php callback may write stats via rpc requests) 2) if yes, binlog is flushed as well as all strings accumulated in "flush_strings_binlog" since the last flush (if it's the first time