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.

- seperate walker and collector in two threads, communicating with queue
- redefining fusefs structure, make it more consitent to fix some bugs

This is consistent with the musicbrainz schema and removes the hacky
code, anyway it is not very useful. Needs more intelligent logic.

Merge branch 'draft'

a more less high level wrapper for llfuse

Merge branch 'draft'