Fix segfault upon exit when in tunnel mode.

* subversion/svnserve/svnserve.c:
  (main): pools containing object references must be cleaned up before the
          root pools / allocators get cleaned up


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1543588 13f79535-47bb-0310-9956-ffa450edef68