mod_ssl_ct: join the threads before their parent pools are destroyed.

mod_ssl_ct: join the threads before their parent pools are destroyed.

This can happen on stop/restart for the daeomon thread, or on clean_child_exit()
for the service thread.

When an apr_thread_create()d thread exits it destroys its pool (in any case),
either explicitely when apr_thread_exit() is called, or implicitely after the
function returns (only in APR 2.0 for now).

So we should make sure that mod_ssl_ct's daemon and service threads exit before
pconf and pchild (the parent pools, respectively) destroy their children pools,
otherwise the threads' pool will be destroyed twice and cause a crash.

Using a pre_cleanup to wait for the threads avoids this.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1883667 13f79535-47bb-0310-9956-ffa450edef68