Consolidate delivery buffer flushing of all route types

Consolidate delivery buffer flushing of all route types

Route::no_roll(), Route::roll(), Track::no_roll(), AudioTrack::roll()
and MidiTrack::roll() all had the exact same loop for flushing buffers
of their Delivery processors. That was a lot of replicated code that had
to be kept synchronised by hand. Put that code into a protected method
Route::flush_processor_buffers_locked() which is called instead.