Update DPKI document, section 6 — identity lifecycle/recovery

[taoeffect]

Right now section 6 puts too much focus on shamir's secret sharing. Our understanding of how identities are managed and recovered has since evolved and we need to be clear about the best way(s) of managing the lifecycle and representation of an identity, e.g. via an identity contract, where identity recovery/replacement is simply done via n-of-m signatures from identities that user trusts. No secret sharing needed/wanted.