[Research][Post-Quantum] Key rotation and TTL's for keys (counter)

[FantasticoFox]

@phil-jakubeit came up with the idea to set a pre-defined TTL for every key how often they are allowed to sign before they are replaced by a successor which is instantiated by the last signature of the predecessor.

Turning a vulnuarbility into a feature, to ensure rolling key rotations to mitigate risks of private key leakage and also support future resiliance towards post quantum cryptography.

Foowing a core principle: a system must have a death function / cleaning function which leads to a pressure to renew it's components.

See also Recommendation for Stateful Hash-Based Signature Schemes for tracking how many signatures has been made. "eXtended Merkle Signature Scheme (XMSS) and the Leighton-Micali Signature system (LMS) "