(Bug 20744) - Wiki forgets about an uploaded file

(Bug 20744) - Wiki forgets about an uploaded file
(Bug 24978) - "A file disappeared" thread on Commons:VP
Only save the LocalFile to memcache after the db entry has been commited.
Otherwise, when the transaction fails, the file apparently succeeded, but
after a week (memcached expiry) it disappears.
Fixes bug 20744 and bug 24978. Found by the Great Domas.

The correlation with db problems is probably just that in such case there are more transactions get rollbacked.