add transactional versions of some API calls

bramble-api/src/main/java/org/briarproject/bramble/api/contact/ContactManager.java

@@ -107,6 +107,12 @@ public interface ContactManager {
 	 */
 	String getHandshakeLink() throws DbException;
 
+	/**
+	 * Returns the handshake link that needs to be sent to a contact we want
+	 * to add.
+	 */
+	String getHandshakeLink(Transaction txn) throws DbException;
+
 	/**
 	 * Creates a {@link PendingContact} from the given handshake link and
 	 * alias, adds it to the database and returns it.