Introduce a state machine for Bob's secure-join

This introduces a state machine which takes care of managing the
handshake transitions in the secure-join protocol.  This separates
user interactions from the protocol state handling.

This means that while handling the protocol state there are a bunch of
failures no longer possible due to all state information being
guaranteed to be present.  As part of this the QR-code state has been
extracted from the generic Lot structure to something suitable just
for the SecureJoin protocol.

A LogSink has been added to the testing tools allowing log messages to
be correctly displayed on test failures.

src/dc_receive_imf.rs

@@ -440,7 +440,6 @@ async fn add_parts(
                 }
                 Err(err) => {
                     *hidden = true;
-                    context.stop_ongoing().await;
                     warn!(context, "Error in Secure-Join message handling: {}", err);
                     return Ok(());
                 }