MINOR: config: make the stream interface idle timer user-configurable

The new tune.idletimer value allows one to set a different value for
idle stream detection. The default value remains set to one second.
It is possible to disable it using zero, and to change the default
value at build time using DEFAULT_IDLE_TIMER.
diff --git a/src/stream_interface.c b/src/stream_interface.c
index 83e814c..f23a9b0 100644
--- a/src/stream_interface.c
+++ b/src/stream_interface.c
@@ -1097,7 +1097,8 @@
 	cur_read = 0;
 
 	if ((chn->flags & (CF_STREAMER | CF_STREAMER_FAST)) && !chn->buf->o &&
-	    (unsigned short)(now_ms - chn->last_read) >= 1000) {
+	    global.tune.idle_timer &&
+	    (unsigned short)(now_ms - chn->last_read) >= global.tune.idle_timer) {
 		/* The buffer was empty and nothing was transferred for more
 		 * than one second. This was caused by a pause and not by
 		 * congestion. Reset any streaming mode to reduce latency.