This eliminates some overhead, because the compiler doesn't need to
consider these functions throwing.

Don't open the ZIP file again and again for each song file.

Wildcard matches are directly applied to all filenames in
subdirectories without any attempt at matching relative paths.

This change is based on the following feature request:

  http://bugs.musicpd.org/view.php?id=3729

Necessary for "final" on gcc 4.6.

Prepare for the plugin interface.

Keep only a reference.

Prepare to make this a new plugin API, for example to use a SMB share
for the music_directory.

Decouples db/update/Remove.cpp from global variables.

Remove dependency on the DatabaseSimple library.

Move global variables into the new classes.  That may allow multiple
update threads for multiple databases one day.