remove jlhttp dependency (GPLv2 license)

[mpollmeier]

Having a GPLv2 license in the dependencies limits us downstream
and really isn't required for a simple http server. This replaces
it with HttpServer from the JDK.

jlhttp was initially introduced via #4892

[max-leuthaeuser]

Did you test that with e.g., the CS integrations tests?
Looking at the failed tests here: looks like this is not thread-safe anymore?
server and frontendHTTPHandler were both objects.
(The frontends are mostly not thread-safe, that's why there must be only one request at the time per frontend)

[maltek]

(The frontends are mostly not thread-safe, that's why there must be only one request at the time per frontend)

That's supposed to be opt-in via the SingleThreadedFrontend trait, which forces a single-threaded thread pool.

[max-leuthaeuser]

SingleThreadedFrontend

Ah yeah, totally forgot about that one.