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.

Replaces global variable "main_loop".

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

Don't use the global variable "instance".

Don't use the global variable "main_thread".

Move from db/DatabaseGlue.cxx, eliminating global variable.

Database update was randomly broken due to these uninitialized
variables.

Fixes loading database songs from state file (regression by commit
29072797).

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