This addresses two types of introduction corner cases:

* force decline when two of our own identities are introduced to each
  other
* throw away introduction requests to the same identity
  (impossible to trigger from UI)

Closes #284

briar-api/src/org/briarproject/api/db/DatabaseComponent.java

@@ -102,6 +102,12 @@ public interface DatabaseComponent {
 	 */
 	boolean containsGroup(Transaction txn, GroupId g) throws DbException;
 
+	/**
+	 * Returns true if the database contains the given local author.
+	 */
+	boolean containsLocalAuthor(Transaction txn, AuthorId local)
+			throws DbException;
+
 	/**
 	 * Deletes the message with the given ID. The message ID and any other
 	 * associated data are not deleted.