Encapsulate RNGState better

Summary:
Partial backport of Core PR14955
https://github.com/bitcoin/bitcoin/pull/14955/commits/cddb31bb0a132afa50b5350196cf26f0064fe3e2

Depends on D4398

Test Plan:

make check

Reviewers: deadalnix, Fabien, jasonbcox, O1 Bitcoin ABC, #bitcoin_abc

Reviewed By: Fabien, O1 Bitcoin ABC, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D4399