Graceful shutdown of the DNSCrypt server
This PR implements Server.Shutdown(ctx context.Context) method that allows
to shut down the DNSCrypt server gracefully.
Some additional changes that were inadvertently made while doing that:
1. Added benchmark tests
2. Started using dns.ReadFromSessionUDP / dns.WriteToSessionUDP instead of implementing it by ourselves
3. Generally improved tests
4. Added depguard
5. Improved comments overall in the code
