Cached annotations should be distributed into subdirectories

[Nicklas Nordborg]

The annotation cache implemented in #1374 saves the data in files in the static cache. Currently all files are stored in the same directory. We just noticed that there are over 600 000 annotated items on the production server, which means that the same number of files may be created in the cache directory. This may not be a good idea since it seems like some file systems doesn't handle this very well. I think we should distribute the cached files into subdirectories. This should be a relatively easy to fix. The cache key is created by SnapshotManage.getCacheKey() and we can use the ID to generate a subdirectory path. We don't have to care about the existing files. New ones will be created if needed and the old ones will automatically be deleted after some time.

[Nicklas Nordborg]

(In [5172]) Fixes #1423: Cached annotations should be distributed into subdirectories