Problem:
--------
InnoDB slow shutdown doesn't work because of background threads
(fts_optimize_thread, dict stats thread) adding undo to purge after/while
the purge thread is exiting.

Fix:
----
Exit the background threads like fts_optimize_thread, dict stats thread
before intiating purge shutdown.

Also added debug assert which will catch if any thread is trying to add
undo to purge history list after the purge thread is finished.

Reviewed-By: Marko Makela <marko.makela@oracle.com>
RB: 11046