pulse: code rewrite using the asynchronous libpulse API

This is a complete rewrite of the PulseAudio output plugin.  It uses
the asynchronous API, which gives us more control over everything.
Additionally, it connects to the PulseAudio server on startup, and
keeps this connection up while MPD runs.  During pause, instead of
closing the stream, it enables "cork".

test/run_output.c

@@ -24,6 +24,7 @@
 #include "audio_parser.h"
 #include "filter_registry.h"
 #include "pcm_convert.h"
+#include "event_pipe.h"
 
 #include <glib.h>
 
@@ -31,6 +32,11 @@
 #include <string.h>
 #include <unistd.h>
 
+void
+event_pipe_emit(G_GNUC_UNUSED enum pipe_event event)
+{
+}
+
 void pcm_convert_init(G_GNUC_UNUSED struct pcm_convert_state *state)
 {
 }