Enhance the communication filtering mechanism

* Add an extra argument to the filter: a smx_action_t describing the
  other side's communication. That's useful to write filters on other
  side's identity
* Ensure that if no comm matches the filters, the filter function gets
  saved in the comm object that gets created and stored in rdv point.
  Later on, when someone else tries to make a match, use that stored
  function to ensure that the stored filter accepts to match the new
  commer.

Thanks to Sorina Camarasu for exploring all this.