Client for negotiating supported clients

Closes #237

See merge request akwizgran/briar!759

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