Marek Vavruša authored Jul 31, 2018 and Petr Špaček committed Dec 05, 2018

This commit allows opportunistic DNS over TLS to origins configured
as supporting DoT on port 853. It also adds interface for clearing
configured TLS clients to allow runtime reconfiguration.

The general mode of operation is as follows:

1. Produce a new outgoing query
2. Check if the selected upstream address has configured TLS support on port 853
 2a. If it does: upgrade to DNS over TLS, it cannot be downgraded from this point
 2b. If not: continue with preferred protocol

This allows further automatic discovery as in [1], but right now it has to be configured
manually.

[1]: https://tools.ietf.org/id/draft-bortzmeyer-dprive-resolver-to-auth-00.html

(cherrypicked from cloudflare branch, need to be adapted)