Replace references to bitcoin mailing lists

[cornelius]

There are a couple of references to the bitcoin mailing lists, e.g. in doc/dnsseed-policy.md. Clonemachine has turned them into a link to a unit-e mailing list which does not exist. We should replace them by working links once we have a communication channel in place which can serve for this purpose (see https://github.com/dtr-org/unit-e-docs/issues/41).