added support for detecting "normal" shutdowns and sending notifications...

It's useful to have a lifecycle listener also report when shutdown events appear to be in progress, although this can be tricky depending on exacting timing and circumstances. I'm not entirely sure what time is available allotted during a normal shutdown after each `before`, `during` and `after` stop event, but my tests indicate this PR works "acceptably".

It obviously does nothing for _abnormal_ shutdowns, but it's still a bit more proactive vs reactive to catch the nominal cases (i.e. reduce the need for rapid fire active health checks against tomcat ports).