Fix: Wait for terminated threads to join in thread_pool::Impl::resize

[LazyDodo]

Description

Part of solving #3851.

When decreasing the number of threads in thread_pool::Impl::resize it was possible for the threads marked for termination to still be fully alive at process exit time if the call to resize and the exit of the main function were close enough to each other.

This change makes resize wait for the threads to actually join guaranteeing the threads have been shut down in a orderly manner when the resize function exits.

Tests

N/A

Checklist:

• I have read the contribution guidelines.
• If this is more extensive than a small change to existing code, I
  have previously submitted a Contributor License Agreement
  (individual, and if there is any way my
  employers might think my programming belongs to them, then also
  corporate). - small change no CLA required.
• I have updated the documentation, if applicable. - N/A
• I have ensured that the change is tested somewhere in the testsuite N/A
  (adding new test cases if necessary).
• My code follows the prevailing code style of this project. - Just moved code around should have no style issues