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.

Note: this is exactly D16728. I managed to land a previous revision by mistake (see D16772) so I need to resubmit the diff.

Test Plan:

ninja check

Reviewers: #bitcoin_abc, PiRK, roqqit

Reviewed By: #bitcoin_abc, PiRK, roqqit

Differential Revision: https://reviews.bitcoinabc.org/D16773