Allow read-only transaction when not marking message as sent.

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

@@ -492,6 +492,8 @@ public interface DatabaseComponent extends TransactionManager {
 	 * Returns the message with the given ID for transmission to the given
 	 * contact over a transport with the given maximum latency. Returns null
 	 * if the message is no longer visible to the contact.
+	 * <p/>
+	 * Read-only if {@code markAsSent} is false.
 	 *
 	 * @param markAsSent True if the message should be marked as sent.
 	 * If false it can be marked as sent by calling

bramble-core/src/main/java/org/briarproject/bramble/db/DatabaseComponentImpl.java

@@ -746,7 +746,9 @@ class DatabaseComponentImpl<T> implements DatabaseComponent {
 	public Message getMessageToSend(Transaction transaction, ContactId c,
 			MessageId m, long maxLatency, boolean markAsSent)
 			throws DbException {
-		if (transaction.isReadOnly()) throw new IllegalArgumentException();
+		if (markAsSent && transaction.isReadOnly()) {
+			throw new IllegalArgumentException();
+		}
 		T txn = unbox(transaction);
 		if (!db.containsContact(txn, c))
 			throw new NoSuchContactException();