Use svn_config_t.x_pool only for data that gets (temporarily) stored
in our config tree.  This will allow us to not create the x_pool for
r/o configs at all because they are always fully expanded.

* subversion/libsvn_subr/config_impl.h
  (svn_config_t): update commentary

* subversion/libsvn_subr/config.c
  (svn_config_get,
   svn_config_enumerate): use cfg->pool instead of x_pool as bases for
                          temporary pools

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1538072 13f79535-47bb-0310-9956-ffa450edef68