Compute the real-time hash target for the next block

Summary:
Given some input parameters, compute the next minimum hash for a block to be avalanche accepted.
Only the computation is part of this diff.

Inspired by the research from https://ledger.pitt.edu/ojs/ledger/article/download/195/187/1008.

Test Plan:

ninja check

Reviewers: #bitcoin_abc, PiRK, roqqit

Reviewed By: #bitcoin_abc, PiRK, roqqit

Subscribers: bytesofman

Differential Revision: https://reviews.bitcoinabc.org/D16728