There are threading problems with tile cache usage

There are two problems with the tile cache which appear to be related to threading.

The first is cache corruption - occasionally tiles will be saved with the wrong filename, which means the tile will be drawn in the wrong place until it is evicted from the cache. The occasional nature makes it look like it might be a race condition somewhere, which indicates there is probably a flaw with how the threading is organised and executed.

The second is that there are occasionly crashes on startup related to accessing some internal data used to manage the caches and plugins and the tiles being requested for them. This appears to be a race condition as well.