BUG/MINOR: task: Don't defer tasks release when HAProxy is stopping

BUG/MINOR: task: Don't defer tasks release when HAProxy is stopping

A running or queued task is not released when task_destroy() is called,
except if it is the current task. Its process function is set to NULL and we
let the scheduler to release the task. However, when HAProxy is stopping, it
never happens and some tasks may leak. To fix the issue, we now also rely on
the global MODE_STOPPING flag. When this flag is set, the task is always
immediately released.

This patch should fix the issue #1714. It could be backported as far as 2.4
but it's not a real problem in practice because it only happens on
deinit. The leak exists on previous versions but not MODE_STOPPING flag.