Refine the logic around how to handle hard vs soft bounce vs compaint

[brendanheywood]

https://github.com/catalyst/moodle-tool_emailutils/blob/master/client.php#L48-L58

https://aritic.com/blog/aritic-mail/practices-to-avoid-most-email-bounce/

We can't rely on the core logic in over_bounce_threshold, so I think we want something like:

a) if its a hard bounce, set emailstop as well because we want immediate stopped emails

https://tracker.moodle.org/browse/MDL-14350

b) if its a soft bounce then we want to gradually decide if we should stop

etc all the way through that table