Improve parent-tile handling when some 404s are present

[tmcw]

We're running into problems with sparse tilesets in which a single 404 can block the removal of a parent tile, leading to the awful sustained-blurry-big tile problem. Should take a closer look at this code to come out with a better averaging of the two priorities:

1. Don't flash white no-tiles content when zooming in / out
2. Remove the parent tile when all requests have finished, even if they were in error

This problem might lead to a solution for #16 as well.