Create a better mechanism for outputting progress information from batch jobs

I hate the sort of

if ($feedback) {
echo ...;
}

or

if (!defined('RESTORE_SILENTLY'))

{ echo "<ul>"; }

code we have around the place.

Instead, I am going to make a moodle_progress_trace class with a single output($message) method, with subclasses like null_progress_trace, text_progress_trace, html_progress_trace, etc. Then the batch code can just do $feedback->message('...'); and the right thing will happen.