Add database method for retrieving a contact by author ID

and use it for retreiving the status of an author faster.
Also add tests for both.

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

@@ -176,6 +176,14 @@ public interface DatabaseComponent {
 	 */
 	Collection<Contact> getContacts(Transaction txn) throws DbException;
 
+	/**
+	 * Returns a possibly empty collection of contacts with the given author ID.
+	 * <p/>
+	 * Read-only.
+	 */
+	Collection<Contact> getContactsByAuthorId(Transaction txn, AuthorId remote)
+			throws DbException;
+
 	/**
 	 * Returns all contacts associated with the given local pseudonym.
 	 * <p/>