swarm: correctly avoid multiple 1:1 with same contact

In the contact list, receiving a confirmed request for a non-active
contact would re-add the contact.  If the contact was removed then
re-added, the conversation would be badly updated & removed, instead
of replacing the conversation by the old one.

Finally, remove the replaced conversation after the clone of the
new one, to avoid making the contact disappear while cloning.

Add some tests to validate the scenarios.

Change-Id: I67a94604c5869804ed66b390aa26cca9420fbc28