1. bulk scan begins transaction, locking the db
2. cache regeneration is triggered, but waits for db to unlock
3. bulk scan calls cache_artwork_ping, which can't return because cache thread is waiting
-> scan thread is waiting for cache thread, which is waiting for scan thread
- introduced new section in config file for sqlite pragma settings,
added config for artwork cache
- added setting of pragma synchronous, cache size and journal mode to
daap cache
