init: use std::thread for ThreadImport()
init: use std::thread for ThreadImport()

Summary:
Mentioned in [[https://github.com/bitcoin/bitcoin/pull/19142 | core#19142]], which removed the boost::interruption_point()
in ThreadImport().

This is a backport of [[https://github.com/bitcoin/bitcoin/pull/19197 | core#19197]]

Test Plan: `ninja all check-all`

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D9833

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