Explicitly terminate the thread pool in svnserve.

Just like in our latest change to the C test framework, svnserve uses
APR thread pools.  Hence, we might run into the same cleanup ordering
issue that we observed in the tests.

* subversion/svnserve/svnserve.c
  (sub_main): Always initialize the THREADS variable.
  (main): If thread pools were used, make sure to empty them
          before destroying memory pools.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1545943 13f79535-47bb-0310-9956-ffa450edef68