Speak to keyservers directly and drop dirmngr

We were discussion to maybe speak to keyservers directly, via HTTPS.

Pro:

• We can get rid of a lot of shell-calls,
• it it much easier and probably much more realiable to test, sks-mock.rb and repeatingly killing dirmngr could probably be dropped,
• we still can support multiple, specific keyservers,
• we have much more control over the requests, e.g. can retry in case of timeouts.

Contra:

• We can not support keyserver-pools (because they don't use proper HTTPS-certificates),
• we must use a hardcoded default, can't fall back to a system-wide configured keyserver,
• supporting connections via TOR requires additional work (but not that much: https://rubygems.org/gems/socksify, https://stackoverflow.com/questions/13353544/ruby-tor-and-nethttpproxy/13882749#13882749)