[dnsdist] dnsdist package from master repo (2019-05-09)

[Christoph]

> In the buster development cycle there was a gcc issue that affected
> a number of C++ programs, including dnsdist (and pdns-recursor).
> This was "solved" in Debian by having libstdc++6 break the older
> versions, but that clearly breaks the very reasonable special
> versioning of the repo.powerdns.com master-branch packages.

thanks for this background information

> This is tracked as https://github.com/PowerDNS/pdns/issues/7781

looking forward to see the mentioned fix getting merged!