refactor: use std::shared_mutex & remove Boost Thread

Summary:

refactor: replace Boost shared_mutex with std shared_mutex in cuckoocache tests

refactor: replace Boost shared_mutex with std shared_mutex in sigcache

build: don't build or use Boost Thread

Co-authored-by: MarcoFalke falke.marco@gmail.com
Co-authored-by: sinetek pitwuu@gmail.com

This is a backport of core#21064
Depends on D13963

Test Plan:
ninja check
gitian builds

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D13966