Switched sync.{cpp,h} to std threading primitives.

Summary:
threads: add a thread_local autoconf check

This is a backport of Core PR11722

Depends on D1901 and D1902

Test Plan:

make check
ninja all check

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1903