Introduce a MessageContext class for more flexibility

This change will allow to pass message dependencies from the client
validators to the ValidationManager.

briar-api/src/org/briarproject/api/sync/BaseMessageContext.java

@@ -0,0 +1,17 @@
+package org.briarproject.api.sync;
+
+import java.util.Collection;
+
+public abstract class BaseMessageContext {
+
+	private final Collection<MessageId> dependencies;
+
+	public BaseMessageContext(Collection<MessageId> dependencies) {
+		this.dependencies = dependencies;
+	}
+
+	public Collection<MessageId> getDependencies() {
+		return dependencies;
+	}
+
+}