output/alsa: use a new I/O thread with real-time scheduling

The normal I/O event thread can have a large latency, e.g. when
libgnutls loads all TLS CA certificates for a https connect.  This
makes it unreliable for the ALSA I/O notifications, and causes ring
buffer xruns.  To avoid interfering with high latency events such as
CURL's, we move the ALSA I/O events to a separate I/O thread which
also obtains real-time scheduling (if possible).

Closes #221

src/Instance.hxx

@@ -76,8 +76,19 @@ struct Instance final
 	, public RemoteTagCacheHandler
 #endif
 {
+	/**
+	 * A thread running an #EventLoop for non-blocking (bulk) I/O.
+	 */
 	EventThread io_thread;
 
+	/**
+	 * Another thread running an #EventLoop for non-blocking
+	 * (real-time) I/O.  This is used instead of #io_thread for
+	 * events which require low latency, e.g. for filling hardware
+	 * ring buffers.
+	 */
+	EventThread rtio_thread;
+
 	MaskMonitor idle_monitor;
 
 #ifdef ENABLE_NEIGHBOR_PLUGINS