Fixed ThreadPool when there are too many inactive threads #4485

[alexey-milovidov]

For changelog. Remove if this is non-significant change.

Category (leave one):

• Bug Fix

Short description (up to few sentences):
When there are more than 1000 threads in a thread pool, std::terminate may happen on thread exit. The bug and the fix is described by Azat Khuzhin in PR #4485.

[alexey-milovidov]

Without this PR the test output is:

$ ./thread_pool_3
.......terminating
..Aborted (core dumped)