Also flush buffers of the inner delivery of inserts

Also flush buffers of the inner delivery of inserts

When flushing the buffers of Delivery processors owned by a Route/Track,
inner deliveries of PortInsert processors were missed since PortInsert
is not a Delivery subclass, but rather owns a Delivery as a private
member. Expose a flush_buffers() for PortInsert and call it too.

This is correct since (external) Send is a Delivery subclass, so that
just makes the send part of inserts behave as external sends do.