Abstract out seeding/extracting entropy into RNGState::MixExtract

Summary:
Partial backport of Core PR14955
https://github.com/bitcoin/bitcoin/pull/14955/commits/2ccc3d3aa346e96206281a391bc29874cf5ee7f4

Depends on D4392

Test Plan:

make check

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

Reviewed By: jasonbcox, O1 Bitcoin ABC, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D4393