When uploading a new file, save to memcached directly after commit

When uploading a new file, save to memcached directly after commit

Previously if an exception was thrown when creating the description
page, the old (negative) memcached entry would not be purged, and
the file would appear not to exist until 24 hours later.

This way, as soon as we have the image row committed to db, we
change the memcache entry, so memcache of image row always looks
like actual image row.

Bug: 32551
Change-Id: I9c260e9a4a1e0d6ade1771542dd834f969904cef