In commit 360a4a03
some debug assertions were introduced to the page flushing code
in XtraDB. Add these assertions to InnoDB as well, and adjust
the InnoDB shutdown so that these assertions will not fail.

logs_empty_and_mark_files_at_shutdown(): Advance
srv_shutdown_state from the first phase SRV_SHUTDOWN_CLEANUP
only after no page-dirtying activity is possible
(well, except by srv_master_do_shutdown_tasks(), which will be
fixed separately in MDEV-12052).

rotate_thread_t::should_shutdown(): Already exit the key rotation
threads at the first phase of shutdown (SRV_SHUTDOWN_CLEANUP).

page_cleaner_sleep_if_needed(): Do not sleep during shutdown.
This change is originally from XtraDB.