Gapfilling Mode

There should be a receive_gapfilling(..., fill_value) mode with the following traits:

• If an expiry date approaches and the data is not there, a "zero" packet is delivered.
• The filling pattern (e.g. "zero" should be configurable on the receive call).

ToDo:

• Think about how to predict an "expiry" of a packet that has not been sent or recovered.