rework MIDI [processor|plugin] chain

* forward midi-data around plugins that have no MIDI-out
* allow to insert plugins with no MIDI-input at a point with one MIDI-channel

This works because excess ports (both plugin and route) remain
unconnected and use scratch-buffers.

Tested with LV2, LXVST and LADSPA.
(AU plugins with variable in/out retain the old behavior, no bypass)

fixes http://tracker.ardour.org/view.php?id=5630