avoid lockup in `ThreadPool::shutdown()` on legacy host (#1417)
avoid lockup in `ThreadPool::shutdown()` on legacy host (#1417)

It is found on CentOS 7.0.1406 which is very early version of CentOS 7.

Co-authored-by: Changbin Park <2f738811815602a0912ea07fd40355a22f885d76@ahnlab.com>
yhirose/cpp-httplib,yhirose/cpp-httplib,yhirose/cpp-httplib,yhirose/cpp-httplib