Remove unused boost/thread

Summary:

txindex: Remove unused boost/thread

txdb: Remove unused boost/thread

refactor: Specify boost/thread/thread.hpp explicitly

There are predefined interruption points for boost::thread: https://www.boost.org/doc/libs/1_71_0/doc/html/thread/thread_management.html#interruption_points

However, non-boost threads such as std::thread or the main() thread can obviously not be interrupted. So remove all unused boost/thread from methods that are never executed in a boost::thread.

Most of them were accompanied by a ShutdownRequested anyway. So even if the current thread was a boost::thread, the interruption point would be redundant. (We only interrupt threads during shutdown)

To make it work for our codebase, I had to add an addition include in system.cpp and core_io_tests.cpp.

This is a backport of core#18758

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, majcosta, deadalnix

Reviewed By: #bitcoin_abc, majcosta, deadalnix

Subscribers: majcosta

Differential Revision: https://reviews.bitcoinabc.org/D9522