Introduction protocol fails for one introducee
Looks like we better rewrite the introduction client: Tried an introduction, both introducees accepted, contact was added for one, but not the other.
I could reproduce this by triggering an abort with #893 (closed). When the first response came in already, the second introduce accepts and sends the ACK directly. Then the introduce accepts taps ACCEPT again which aborts the session, so the contact will not be added. However, the first introducee already received response and ACK and added the contact unilaterally.
What is worse, when aborting the second introducee fail deleting the inactive contact which prevents that contact from ever being added in the future.