Fix the calculation of the next payday time in GetTokenomicsInfo

[td202]

Purpose

Closes #1240

This fixes the timing information for the next payday returned by the GetTokenomicsInfo query. Instead of basing it on the genesis block time, the time is computed based on the next epoch transition time. This is more accurate, since after the P6->P7 update, the first epoch (epoch 0) is already considered complete.

Changes

• Revise the computation of the next payday time, as described above.
• Bump node version for new release.

Checklist

• My code follows the style of this project.
• The code compiles without warnings.
• I have performed a self-review of the changes.
• I have documented my code, in particular the intent of the
  hard-to-understand areas.
• (If necessary) I have updated the CHANGELOG.

[drsk0]

Looks good, thanks!