Tracking issue for NRD kernels

[lehnberg]

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

• https://github.com/mimblewimble/grin-rfcs/pull/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 (https://github.com/mimblewimble/grin/pull/3302)
• [x] introduce new NRD kernel variant, (specifically ser/deser) (https://github.com/mimblewimble/grin/pull/3303)
• [ ] NRD "rules" - implement NRD "lookback" logic (based on kernel pos index) [tbd]
• [ ] wallet changes? (Do we support these at all initially?)
  • [ ] https://github.com/mimblewimble/grin-wallet/issues/393 (issue)
  • [ ] https://github.com/mimblewimble/grin-wallet/pull/408 (wip PR)
• [ ] hardfork
  • [ ] floonet hardfork
  • [x] block validity (including NRD kernel pre and post HF height) (https://github.com/mimblewimble/grin/pull/3303)
  • [x] txpool/stempool - do we broadcast transactions containing NRD kernels? ~- [ ] filter txpool buckets when preparing block to mine(https://github.com/mimblewimble/grin/pull/3318 + additional NRD rule)~
• [x] "half kernels" - we should be able do this with plain kernels to demonstrate (https://github.com/mimblewimble/grin/pull/3314)
  • [x] build 2 kernels sharing same excess commitment
  • [x] build 2 transactions with intermediate output