Touch mirrors on even on fail to update (#19217)

Touch mirrors on even on fail to update (#19217)

* Touch mirrors on even on fail to update

If a mirror fails to be synchronised it should be pushed to the bottom of the queue
of the awaiting mirrors to be synchronised. At present if there LIMIT number of
broken mirrors they can effectively prevent all other mirrors from being synchronized
as their last_updated time will remain earlier than other mirrors.

Signed-off-by: Andrew Thornton <art27@cantab.net>