[C++] An unhandled race condition exists in ThreadPool

[asfimport]

A race condition exists at the ThreadPool which may lead to the loss of pending tasks after a process forks. 

See this issue for more detail: #12329

Environment: linux
Reporter: Huxley Hu
Assignee: Antoine Pitrou / @pitrou

Externally tracked issue: #12329

PRs and other links:

• GitHub Pull Request #12350
• GitHub Pull Request #12358

_{Note: This issue was originally created as ARROW-15593. Please see the migration documentation for further details.}

[asfimport]

Yibo Cai / @cyb70289:
Issue resolved by pull request 12358
#12358