Fix: manually calculate current reward cycle ID in /v2/pox_info

[kantai]

Description

/v2/pox invokes get-pox-info in the current PoX contract to obtain information about the current cycle. However, the burn view of this invocation is always one block behind the current chain tip: 2.x Stacks blocks are evaluated with a burnchain view equal to their parent (because the Stacks blocks must be assembled before the burn block in which they are committed).

This leads to an "off by one" situation at reward cycle boundaries. This isn't a huge problem for Stacks 2.x nodes, because the information returned from this endpoint isn't really critical during the reward cycle change-overs. For signers in Nakamoto, though, this is problematic: they must know that the reward cycle has changed because they are now responsible for signing any new blocks at the time of the change-over.

This PR addresses this problem by manually computing the current reward-cycle ID.

[codecov]

Codecov Report

Attention: Patch coverage is 83.33333% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 82.38%. Comparing base (4530f01) to head (d5af26e).
Report is 36 commits behind head on next.

Additional details and impacted files

@@            Coverage Diff             @@
##             next    #4468      +/-   ##
==========================================
- Coverage   83.45%   82.38%   -1.07%     
==========================================
  Files         448      453       +5     
  Lines      323993   325564    +1571     
  Branches        0      318     +318     
==========================================
- Hits       270387   268227    -2160     
- Misses      53606    57329    +3723     
- Partials        0        8       +8     

Files	Coverage Δ
stackslib/src/net/api/getpoxinfo.rs	93.33% <83.33%> (-0.58%)	⬇️

... and 59 files with indirect coverage changes

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[obycode]

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