input_stream: non-blocking I/O

Add GMutex, GCond attributes which will be used by callers to
conditionally wait on the stream.

Remove the (now-useless) plugin method buffer(), wait on GCond
instead.  Lock the input_stream before each method call.  Do the same
with the playlist plugins.

src/text_input_stream.c

@@ -68,8 +68,8 @@ text_input_stream_read(struct text_input_stream *tis)
 	do {
 		dest = fifo_buffer_write(tis->buffer, &length);
 		if (dest != NULL) {
-			nbytes = input_stream_read(tis->is, dest, length,
-						   &error);
+			nbytes = input_stream_lock_read(tis->is, dest, length,
+							&error);
 			if (nbytes > 0)
 				fifo_buffer_append(tis->buffer, nbytes);
 			else if (error != NULL) {