Pox: Disallow stacking locks during prepare phase

[friedger]

Is your feature request related to a problem? Please describe.
As a stacker, I lock my stacks before the start of the cycle but after the start of the prepare phase and my stx do not earn any rewards

Describe the solution you'd like
Currently, there is no way to learn about the cycle anchor block. It would be nice to include that information in an api.
Furthermore, the stacking transactions with (delegate-) stack-stx, (delegate-) stack-extend/increase should fail if called during the prepare phase.

additional context
See "Not earning locked stx" table on https://stacksonchain.com/dashboards/Fast%20Pool/276

The discussion on discord led to this feature request: https://discord.com/channels/621759717756370964/730814571517968436/1112787020117463100

[friedger]

Added check-not-prepare-phase in https://github.com/stacks-network/stacks-blockchain/tree/feat/pox-4

[friedger]

Blocked by #3976

[friedger]

There is a discussion for this topic: #4193

[kantai]

This wasn't implemented in pox-4, which means this feature request, if implemented, would require a pox-5. This means that it cannot be 3.0 activated, so removing the nakamoto tags from this for now.