This allows it to be replaced for testing.

These don't affect client visibility.

Hooks are now called exactly once per contact.

Update transport keys in-place to retain key set IDs

Closes #1213

See merge request akwizgran/briar!779

Don't make or accept Bluetooth contact connections during key agreement

See merge request akwizgran/briar!770

IntroduceeProtocolEngine uses wrong role when adding keys

See merge request akwizgran/briar!780

Don't automatically respond to declined introductions

See merge request akwizgran/briar!777

Fix introduction response messages in UI and some minor fixes

Closes #923

See merge request akwizgran/briar!776

Send automatic decline when other introducee declines

See merge request akwizgran/briar!775

New Introduction Protocol

Closes #308, #377, #474, and #613

See merge request akwizgran/briar!758

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.