[3 of 5] Move CDiskBlockPos from chain to flatfile.

Summary:
partial PR15118 backport
https://github.com/bitcoin/bitcoin/pull/15118/commits/d6d8a78f26f52fdfe43293686135e2fc6919926c

Backport note: something similar in spirit was done in D1655, but
conflicts with how Core ended up doing it. This overrides that change
and syncs to how Core does it currently. The class itself is unchanged
except for the ToString method being moved to cpp file.

Depends on D3586

Test Plan: make check

Reviewers: deadalnix, jasonbcox, Fabien, nakihito, #bitcoin_abc

Reviewed By: jasonbcox, nakihito, #bitcoin_abc

Maniphest Tasks: T631

Differential Revision: https://reviews.bitcoinabc.org/D3587