chg: test: asyncserver.py: TCP improvements

This branch started off as `michal/upforwd-asyncserver`.  It quickly
turned out that the critical `asyncserver.py` change that was needed for
the `upforwd` system test was for the server to be able to read multiple
TCP queries on a single connection.  As currently present in `main`,
`asyncserver.py` closes every client connection after servicing a single
query.  Retaining that behavior would cause the `upforwd` system test to
fail and, in general, capturing all data sent by a client seems more
useful in tests than just closing connections quickly.  `asyncserver.py`
can always be extended in the future (e.g. by adding a new
`ResponseAction` that the networking code would react to) to reinstate
the original behavior, if it turns out to be necessary.

While working on changing that particular `asyncserver.py` behavior, I
noticed a couple of other deficiencies in the TCP connection handling
code, so I started addressing them.  One thing led to another and before
I noticed, enough changes were applied to be worth doing a separate
merge request, particularly given that the actual rewrite of
`upforwd/ans4/ans.pl` using `asyncserver.py` is trivial once the
required changes to `asyncserver.py` itself are applied.

Merge branch 'michal/asyncserver-tcp-improvements' into 'main'

See merge request isc-projects/bind9!10276