Move cache purging out of doUpdates()

Move cache purging out of doUpdates()

It makes a whole lot more sense to run this at the end of the
update process after post update maintenance rather than just
at the end of schema updates.

Run update.php more than once so all the updatelog entries
are populated and you'll see why it makes sense.

Change-Id: Ice42a31dee1e6b41da4aa0a47e8786579382aff1