Filip Roséen authored Oct 12, 2016 and Hugo Beauzée-Luyssen committed Mar 29, 2017

The following changes refactors the fetcher to take advantange of the
newly introduced background_worker helper. The new implementation
should not only be easier to maintain, but it also adds some
advantages over the old implementation:

 - The implementation has shrunk in size.

 - A fetch-request can include a timeout.

 - Given that there now is a background worker associated with each of
   the different fetcher types (local, network, download):

    - A slow download does not prevent the network-fetcher from
      working the queue.

    - A slow network-fetcher does not prevent further work in regards
      of pending requests in the local fetcher.

 - A fetch request can now be properly cancelled (most importantly
   during VLC close).

 - We no longer invoke modules with "meta fetcher" capability if the
   item already has all metadata in terms of title, album, and artist.

 - We no longer invoke modules with "art finder" capability of the
   item already has vlc_meta_ArtworkUrl.

fixes: #18150

Signed-off-by: Hugo Beauzée-Luyssen <hugo@beauzee.fr>