Consider constrained devices and saving count_q and count_v regularly

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How often must they be stored? Consider Appendix B.1 and saving the SSN.

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See Appendix B.1 for OSCORE: https://tools.ietf.org/html/rfc8613#appendix-B.1

[rikard-sics]

This can wait until issue "Each client can count q up to half(?) the q limit as an optimization" is closed.

[rikard-sics]

Explore further the idea from Carsten mentioned during the CoRE interim on April 28. This was about utilizing the clock (on devices which have one) to not have to save count_q and count_v in some situations.

We can also clarify that saving the counters every N is just a suggestion for implementers. Different solutions may be acceptable. The key point is making sure to not lose or underestimate the counters after reboot (while overestimating them can be fine to some extent).