[2 of 5] validation: Extract basic block file logic into FlatFileSeq class.

Summary:
partial PR15118 backport https://github.com/bitcoin/bitcoin/pull/15118/commits

This combines four refactor commits:
https://github.com/bitcoin/bitcoin/compare/9183d6ef6~1..e0380933e
...and a commit adding unit tests:
https://github.com/bitcoin/bitcoin/pull/15118/commits/4c01e4e159db82ce4b2acce75f709cac996367d7

These have been squashed due to intermediate commits not even building
properly. The tests have been modified to use CDiskBlockPos as the
rename is planned for a subsequent diff. Otherwise, it's the same.

Depends on D3579 D3583 D3584 D3585

Test Plan: make check

Reviewers: deadalnix, Fabien, jasonbcox, #bitcoin_abc

Reviewed By: deadalnix, jasonbcox, #bitcoin_abc

Maniphest Tasks: T631

Differential Revision: https://reviews.bitcoinabc.org/D3586