Fix a pool lifetime issue with object pool container.

The problem is that the owning pool may be the application's
root pool which gets cleaned up *after* other allocators and
their respective root pools.  Since the object pool uses a
separate allocater internally, either pool may receive the
cleanup message first and we must handle both cases.

* subversion/libsvn_subr/object_pool.c
  (svn_object_pool__t): add owning pool
  (root_object_pool_cleanup): new, alternative cleanup function
  (destroy_object_pool): ensure only one cleanup gets executed
  (svn_object_pool__create): update; schedule two alternive cleanups

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1541303 13f79535-47bb-0310-9956-ffa450edef68