Prepare for the plugin interface.

Keep only a reference.

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.

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

The update thread should not affect the rest of the system, therefore
set "idle" priority, and let it only run when nobody else is using the
resources.

Ask the DatabasePlugin for stats when the first client requests them,
not at startup.

Add UpdateQueueItem::id to keep track of the id in every item.
Replaces thhe hack in update_queue_push().

Fixes a few kludges and avoids GLib allocation.

Prepare to migrate away from GLib.  Currently, we're still using GLib
as a backend.

Replaces GLib's GError.

More preparations for multi-player support.

Fixes deprecation warnings.

This is the beginning of multi-player support.  There will be support
for multiple Partition objects in one MPD process.