AudioServer+LibAudio: Make mixing queue-based instead of buffer-based.

Each client connection now sets up an ASBufferQueue, which is basically a
queue of ABuffers. This allows us to immediately start streaming the next
pending buffer whenever our current buffer runs out of samples.

This makes the majority of the skippiness go away for me. :^)

Also get rid of the old PlayBuffer API, since we don't need it anymore.

Libraries/LibAudio/AClientConnection.h

@@ -11,6 +11,5 @@ public:
     AClientConnection();
 
     virtual void handshake() override;
-    void play(const ABuffer&, bool block);
     void enqueue(const ABuffer&);
 };