Support several DNS overrides at once

Currently, `DnsService` only overrides the DNS entry for the ServServ hostname.

In #4, we found that there are several various entries.

So, I propose the following changes.

1. Support overriding of several entries at once in `DnsService`.
2. Support proper rollback of said change (i.e. it should remove all the added entries and replace all the _changed_ ones in the `StopServer` function).
3. It should be done efficiently, in one disk write operation.
4. Add unit tests verifying the new behavior.

You can start work from my dirty changes in branch `feature/multi-dns-override`.