Investigate alternative for Unmarshaler and Marshaler

[bogdandrutu]

The Unmarshaler because of the Unmarshal definition, has an issue when recursively unmarshal other structs that implement the Unmarshaler, see #6029 where the "given" destination to Conf.Unmarshal cannot be checked against the Unmarshaler implementation to avoid a deadlock. The Marshaler seem to not have (I did not carefully check) the same problem, but this needs to be confirmed.

Some possible solutions:

• See yaml.v2 interfaces, they seem to work fine. We may want to switch to that.
• Ignore this issue, document properly in the Conf.Unmarshal the caveat.

[mx-psi]

#9750 would remove this caveat from Conf.Unmarshal, solving this.