fetcher will be used by two threads, making it a module
prevents two caches, locking prevents simultanious execution

- add fetcher_cache_length: maximum cache size
- add fetcher_cache_age: maximum cache age

The cleanup action for maximum cache age is not triggered yet.

- /Releases/<releasegroup>
- /Releases/<releasegroup>/<release>
- /Releases/<releasegroup>/<release>/<track>

- /Artists/<artist>/<releasegroup>
- /Artists/<artist>/<releasegroup>/<release>
- /Artists/<artist>/<releasegroup>/<release>/<track>

All sqlite querys are now exclusive from one instance. This work around
the missing concurrency support of the sqlite module of python. The code
can be removed, once the module will be fixed.