kill the threads in the thread pool in a pre-cleanup

This makes sure that the threads are killed when we start destroying
any sub-pools they may be using.

This is the last missing part to make the test suite run successfully with
full pool debugging enabled.


git-svn-id: https://svn.apache.org/repos/asf/apr/apr/trunk@1460185 13f79535-47bb-0310-9956-ffa450edef68
Add CHANGES entry for pool debugging fixes

PR: 43375, 52785


git-svn-id: https://svn.apache.org/repos/asf/apr/apr/trunk@1460186 13f79535-47bb-0310-9956-ffa450edef68