paaudio: prepare for multiple audiodev

Have a pool of refcounted connections per server, so if the user creates
multiple audiodevs to the same pa server, it will use a single connection.  (It
will still create different streams, so the user can manage those streams
separately in pulseaudio.)

Signed-off-by: Kővágó, Zoltán <DirtY.iCE.hu@gmail.com>
Message-id: d43218f327c62cdbd16ea0c922612025fbc4805e.1566168923.git.DirtY.iCE.hu@gmail.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>