Tracking issue for NRD kernels

[lehnberg]

This is a tracking issue for RFC000X nrd-kernels (aka relative timelocks).

• N(e)RD kernels grin-rfcs#47

As this RFC has not been merged yet, this is a placeholder
issue for the time being, and will be updated if and when
the RFC has been accepted.

Tentative TODO (assuming RFC is accepted) -

• "kernel pos" index (NRD rules and "recent" kernel pos index #3302)
• introduce new NRD kernel variant, (specifically ser/deser) (Enable NRD kernel support (noop) feature flagged #3303)
• NRD "rules" - implement NRD "lookback" logic (based on kernel pos index) [tbd]
• wallet changes? (Do we support these at all initially?)
  • Add support for NRD kernels grin-wallet#393 (issue)
  • NRD kernel variant (compile against grin master) grin-wallet#408 (wip PR)
• hardfork
  • floonet hardfork
  • block validity (including NRD kernel pre and post HF height) (Enable NRD kernel support (noop) feature flagged #3303)
  • txpool/stempool - do we broadcast transactions containing NRD kernels?
    - [ ] filter txpool buckets when preparing block to mine(support explicit filtering of txs when preparing to mine #3318 + additional NRD rule)
• "half kernels" - we should be able do this with plain kernels to demonstrate (add test to demonstrate pair of "half" kernels sharing same public excess #3314)
  • build 2 kernels sharing same excess commitment
  • build 2 transactions with intermediate output