Tentatively move the new config pool API to libsvn_repos.
This is due to our support for URL config paths and the resulting
dependency on libsvn_repos

* subversion/include/private/svn_subr_private.h
  (svn_config_pool__t,
   svn_config_pool__create,
   svn_config_pool__get): move and rename API from here ...

* subversion/include/private/svn_repos_private.h
  (svn_repos__config_pool_t,
   svn_repos__config_pool_create,
   svn_repos__config_pool_get): ... to here

* subversion/libsvn_subr/config_pool.c
  (): moved ...

* subversion/libsvn_repos/config_pool.c
  (): ... here and update due to API renames

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1532244 13f79535-47bb-0310-9956-ffa450edef68