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)