init: use std::thread for ThreadImport()

Summary:
Mentioned in core#19142, which removed the boost::interruption_point()
in ThreadImport().

This is a backport of core#19197

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

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