DoH server rewrite

Version: 5.1.2

Problem: Current DoH server (based on lua-http) is slow and very hard to debug (e.g. #465 (closed)).

Proposed approach: Complete rewrite using a better HTTP library.

Considerations:

• drop HTTP1 support, https://tools.ietf.org/html/rfc8484 recommends HTTP2 anyway:

HTTP/2 [RFC7540] is the minimum RECOMMENDED version of HTTP for use with DoH

• drop support for insecure transport (HTTP-only): Insecure transport complicates design and implementation and has unclear benefits. Let's not implement it before there is clear use-case for it.

• design a extensibility mechanism for kresd modules [#616 (closed)]