Add apr_pool_owner_set function to allow use of pool debugging with threads

Actually this function has been mentioned in the docs for over 10 years
but has never been implemented.

Also consistently destroy the thread's pool when it exits normally, not only
on apr_thread_exit(). This was already done on OS2.

Other platforms than unix are untested.



git-svn-id: https://svn.apache.org/repos/asf/apr/apr/trunk@1460182 13f79535-47bb-0310-9956-ffa450edef68