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/MessageContext.java

@@ -0,0 +1,26 @@
+package org.briarproject.api.sync;
+
+import org.briarproject.api.db.Metadata;
+
+import java.util.Collection;
+
+public class MessageContext extends BaseMessageContext {
+
+	private final Metadata metadata;
+
+	public MessageContext(Metadata metadata,
+			Collection<MessageId> dependencies) {
+
+		super(dependencies);
+		this.metadata = metadata;
+	}
+
+	public MessageContext(Metadata metadata) {
+		this(metadata, null);
+	}
+
+	public Metadata getMetadata() {
+		return metadata;
+	}
+
+}