fix #148: handle responses in SSE or POST

[Fabilin]

This PR resolves the concurrency issue described in #148 by keeping track of which messages have been handled, and handling new ones as they first arrive - no matter the protocol through which they arrive. This not only ensures the loading state is valid, but also that duplicate messages do not get displayed after a page refresh.

The implementation operates on the following assumptions:

1. For any given bot response, the POST response may be processed at any point, before or after any number of SSE messages
2. POST responses always hold the same messages, in the same order, as those sent through SSE
3. POST responses may get dropped due to connection loss
4. SSE messages may get dropped due to connection loss
5. SSE messages always arrive in order
6. If an SSE message is received, all messages before it have been received and processed (at least for that response)
7. SSE messages may arrive after a page reload, due to fix #1557 web_connector: retry sending messages after SSE disconnection tock#1561

Requires theopenconversationkit/tock#1563
Resolves #148