The root WatchDirectory instance already contains the music directory,
and we don't need to fetch it again.

Replaces map_to_relative_path() from Mapper.cxx.

This commit finally allows the database update to scan remote files,
e.g. using the smbclient storage plugin.  However, it is not yet
possible to configure that, therefore the feature is not accessible
yet.

Prepare for the plugin interface.

Keep only a reference.

Fixes bogus assertion failure.

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

Fixes assertion failure when update gets launched during MPD startup.

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

Don't use the global variable "instance".

Don't use the global variable "main_thread".

Database update was randomly broken due to these uninitialized
variables.

Don't use the global variables from the DatabaseSimple library.

Remove dependency on the DatabaseSimple library.

Reduce overhead.

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

Decouple command/OtherCommands.cxx from the "simple" database plugin.