DNS volume 2

[mikz]

• drop all nginx resolver directives from tests
• always use search scopes
• fix parallelelized dns query with no resolvers
• wrapper with resolver for sockets
• simplified resolver initialization

Always use DNS search scopes

In scenarios where there are defined search scopes in /etc/resolv.conf we add one empty one to try to resolve naked domains.

Example: search google.internal travis-worker.internal in /etc/resolv.conf

Docker container started with linked redis container. Query to redis would result in no results
because it would try to find redis.google.internal or redis.travis-worker.internal.
We add empty scope so we also try to resolve the naked domain: redis. which works.