Allow existing contacts to be re-added

Re-adding an existing contact currently throws a ContactExistsException. But re-adding a contact may be necessary if transport properties get out of sync, making it impossible to connect via BTP. If we re-add a contact we should keep any existing private messages and group subscriptions.

If one contact uses the same identity as last time and the other doesn't, they'll disagree about whether it's a re-add, so we can't re-derive the same transport keys. Can we reuse the contact ID (if any), import the new transport properties, delete the old transport keys (if any), and create new transport keys?