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/chat.rs

@@ -3063,7 +3063,7 @@ mod tests {
     #[async_std::test]
     async fn test_chat_info() {
         let t = TestContext::new().await;
-        let chat = t.chat_with_contact("bob", "bob@example.com").await;
+        let chat = t.create_chat_with_contact("bob", "bob@example.com").await;
         let info = chat.get_info(&t).await.unwrap();
 
         // Ensure we can serialize this.