May allow the compiler to optimize calls away.

Add support for the following tags when using the proxy database plugin:
WORK
CONDUCTOR
LABEL
GROUPING
MUSICBRAINZ_WORKID
COMPOSERSORT
ENSEMBLE
MOVEMENT
MOVEMENTNUMBER
LOCATION

Upcoming release of game-music-emu will support it, details here: https://bitbucket.org/mpyne/game-music-emu/pull-requests/23/rsn-support

The current http output doesn't provide a header for cross-origin support. This prevents to use the mpd http stream directly from an other webapplication due the origin from the webpage differs from then the audio stream.

The fix is to add the following header to the http response:
Access-Control-Allow-Origin: *

This commit adds some tags that are (mostly) interesting for listeners
of classical music.

Ensemble
--------
This is an ensemble that is playing the music, such as Wiener
Philharmoniker. The tag can be used to distinguish the ensemble from the
conductor, composer, soloist, and ensemble, that are generally all in
the "ARTIST" tag.

Movement
-------
The movement number and movement (name) of this track, i.e.  "II" and
"Allegro".

ComposerSort
------------
Allows us to look for Beethoven's 9th under B, for Beethoven, not L for
Ludwig.

Location
--------
This is the location of the recording, e.g. "Wiener Musikverein".

Calling consume_all() is illegal in the producer thread.

Fixes bogus Delay() results at the start of playback, because Delay()
thinks the output is paused.

Fixes a bogus assertion failure (which can now be removed).

Instead of generating silence, do nothing, don't waste CPU time.

This code is complicated - and broken: the producer thread is not
allowed to call consumer methods.  Also the code is not necessary
because this plugin implements Interrupt().

OpenDevice was a confusing name because it does not actually open a
device.

Postpone the Start() call until there is something to be played.