Add an IntroductionManager and Validator

This Introduction BSP Client uses its own group to communicate with
existing contacts. It uses four types of messages to facilitate
introductions: the introduction, the response, the ack and the abort.

The protocol logic is encapsulated in two protocol engines, one for the
introducer and one for the introducee. The introduction client keeps the
local state for each engine, hands messages over to the engines and
processes the result and state changes they return.

briar-api/src/org/briarproject/api/event/MessageValidatedEvent.java

@@ -1,5 +1,6 @@
 package org.briarproject.api.event;
 
+import org.briarproject.api.db.Metadata;
 import org.briarproject.api.sync.ClientId;
 import org.briarproject.api.sync.Message;