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