appservice: Replace nginx with aiohttp reverse proxy

nginx does not get along well with unavailable proxy_pass targets -- as
soon as one session pod unexpectedly goes away (crashes, idle timeouts,
networking flakes), it errors out and no session routes can be resolved
any more. There are workarounds [1], but I can't get them to work
properly. Also, nginx does not allow us to hook into (dis)connection
events to implement the session status API (see issue #28).

So it's finally time to grow to something more flexible. Rewrite
multiplexer.py using aiohttp [2], which now handles both our own session
control API as well as the dynamic reverse proxying to the session pods.

This should still be considered as a PoC, but at least there are fewer
moving parts and an easier-to-understand architecture now.

Check behaviour with broken session pods in the tests: Crash a session
pod, and validate that the other sessions still behave correctly.

Fixes #36

[1] https://sandro-keil.de/blog/let-nginx-start-if-upstream-host-is-unavailable-or-down/
[2] https://docs.aiohttp.org/en/stable/index.html

appservice/Containerfile

@@ -1,13 +1,10 @@
 FROM debian:bookworm
 
 RUN apt-get update && \
-    apt-get install -y python3 python3-redis nginx curl inetutils-ping procps && \
+    apt-get install -y python3 python3-redis python3-aiohttp curl inetutils-ping procps && \
     apt-get clean && \
     rm /var/lib/apt/lists/*dists*
 
-# allow unprivileged container user to run nginx and change configuration
-RUN chmod -R a+rw /etc/nginx/ /var/lib/nginx/ /var/log/nginx/ /run
-
 COPY *.py /usr/local/bin/
 
 CMD python3 /usr/local/bin/multiplexer.py