Page cleaner should do LRU flushing regardless of server activity

This is fix for upstream bug
http://bugs.mysql.com/bug.php?id=71988
http://bugs.mysql.com/bug.php?id=70500

page_cleaner should work whether or not there is server activity.
Its iterations become a noop when there is no work to do but we
should not tie it to the server activity.

The page_cleaner thread does spurious background flushing
because of conditional sleep between iterations. The solution
is not to make sleep dependent on server activity etc.