rcpt_to.in_host_list, handle connection.relaying use case

1. read in config file in register, with a callback
   a) config file refreshes when altered
   b) regex only recompiled when loaded and changed (vs every transaction)

2. MAIL FROM domain checked against in_host_list
    sets connection.transaction.notes.local_sender=true if found

3. when relaying=true, rcpt is validated if sender is a local domain
   (before: relaying connections were ignored and subsequently rejected)

4. Includes tests.