Merge lp://staging/~michihenning/thumbnailer/fix-1531038 into lp://staging/thumbnailer/devel

Changed default backlog to 10. Change RateLimiter to work on LIFO basis. Fixes lp:1531038

Changed default backlog to 10. This is necessary because, with a backlog of 20, we ended up never cancelling anything because the sliding window around a list view is smaller than this. In turn, this meant that we always ended up having fewer outstanding requests than the limit, so the cancel messages from the list view always arrived after the request had already gone out on the wire.

Changed the RateLimiter to work on a LIFO basis instead of FIFO. By dealing with the most request first, cancellation has a much better chance of actually cancelling something. In addition, this massively improves the user experience when scrolling through a large collection with a cold cache. Previously, when scrolling down quickly for, say, 150 songs) all the requests for songs that were scrolled over and disappeared off the top of the screen had to complete before the thumbnails for visible region (once scrolling stopped) became visible. With this change, we give priority to the most recent request, so the thumbnails for the visible region where scrolling stops fill in quickly.