Feature: Support persistent connections (Connection: keep-alive).
(#405 by @clue)

This shows a noticeable performance improvement especially when benchmarking
using persistent connections (which is the default pretty much everywhere).
Together with other changes in this release, this improves benchmarking
performance by around 100%.

Feature: Require Host request header for HTTP/1.1 requests.
(#404 by @clue)

Minor documentation improvements.
(#398 by @fritz-gerneth and #399 and #400 by @pavog)

Improve test suite, use GitHub actions for continuous integration (CI).
(#402 by @SimonFrings)