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/decoder/mpcdec_decoder_plugin.c

@@ -61,7 +61,7 @@ mpc_seek_cb(cb_first_arg, mpc_int32_t offset)
 {
 	struct mpc_decoder_data *data = (struct mpc_decoder_data *) cb_data;
 
-	return input_stream_seek(data->is, offset, SEEK_SET, NULL);
+	return input_stream_lock_seek(data->is, offset, SEEK_SET, NULL);
 }
 
 static mpc_int32_t