Add DB method for getting amount of data to sync.

bramble-api/src/main/java/org/briarproject/bramble/api/db/DatabaseComponent.java

@@ -292,6 +292,16 @@ public interface DatabaseComponent extends TransactionManager {
 	 */
 	Message getMessage(Transaction txn, MessageId m) throws DbException;
 
+	/**
+	 * Returns the total length, including headers, of any messages that are
+	 * eligible to be sent to the given contact via a transport with the given
+	 * max latency.
+	 * <p/>
+	 * Read-only.
+	 */
+	long getMessageBytesToSend(Transaction txn, ContactId c, int maxLatency)
+			throws DbException;
+
 	/**
 	 * Returns the IDs of all delivered messages in the given group.
 	 * <p/>