Modernize a-chat

[skade]

This has to be considered a draft.

The 2 major changes are:

1. Move the client to use 2 concurrent tasks and selecting on them for any errors. The tasks are sufficiently diverse to merit 2 tasks.
2. Move the 2 select statements to unified streams and having the broker and the client handle a set of "Events" themselves. This moves their logic down to a nice match.

The stream construction is still a bit ugly, though, especially in the client handling code of the server.