[4 of 5] scripted-diff: Rename CBlockDiskPos to FlatFilePos.

Summary:
partial PR15118 backport
https://github.com/bitcoin/bitcoin/pull/15118/commits/65a489e93d181d3c0f7a9cf79f7c11ff8cf2b0f0

-BEGIN VERIFY SCRIPT-
sed -i 's/CDiskBlockPos/FlatFilePos/g' $(git ls-files 'src/*.h' 'src/*.cpp')
-END VERIFY SCRIPT-

Backport note: I simply re-ran the script, instead of merging the changes.

Depends on D3587

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/D3588