move-only: Move ThreadImport to blockstorage

Summary:
Can be reviewed with the git options
--color-moved=dimmed-zebra --color-moved-ws=ignore-all-space

This is a backport of core#21575 [2/6]
https://github.com/bitcoin/bitcoin/pull/21575/commits/fa413f07a14744e7d7f7746e861aabd9cf938f61

Depends on D11347

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D11348