Refresh the cache in a task queue for notes downloading.

[Todd-Davies]

Code location:
https://github.com/Todd-Davies/appengine-website/blob/master/src/main/java/uk/co/todddavies/website/notes/NotesApiDownloadServlet.java

On notes downloads, add a task to a task queue that updates the download count in the relevant datastore entity and refreshes the memcache key with the new data.