This enables reproducible builds with non-Unicode locales.

* Create at least one contact

This allows it to be replaced for testing.

These don't affect client visibility.

Hooks are now called exactly once per contact.

This will be changed once we have a way to reset state for peers
that were contacts already at some point in the past.
One contact might have deleted the other, but not vice versa.
So they have mismatching state that needs to be reset.

See #2 for more information.

It is possible that a remote DECLINE message arrives short before the
user responds to the introduction.
This will cause a ProtocolStateException which (for now) is just caught
and a generic (existing) error message will be shown.