References
[1] https://tools.ietf.org/html/rfc7540
[2] https://queue.acm.org/detail.cfm?id=2716278
[3] https://www.imperva.com/docs/Imperva_HII_HTTP2.pdf
[4] https://ilia.ws/files/zend_performance.pdf
[5] https://varnish-cache.org/docs/trunk/phk/firstdesign.html
[6] https://trends.builtwith.com/web-server, March 2018.