Introduce node serialization strategy.

Introduce node serialization strategy.

With the addition of reminders, we may encounter scenarios where
either a bit flip may suffice, instructing the client to look up its
latest data, or we may need to serialize all announcement details
a new if the client may have already purged the old data.

To better distinguish between these scenarios, we introduce a
serialization strategy enum that allows serializing either the full
announcement, just the mutations, or serve solely as a reminder and
serialize nothing at all.