Fix various issues with the config_pool object that came up during testing.

* subversion/libsvn_repos/config_pool.c
  (config_pool_cleanup,
   config_ref_cleanup): make sure only one thread acquires the cleanup token
  (config_by_url): use our specified hash function
  (svn_repos__config_pool_create): allocate in the right pool
  (svn_repos__config_pool_get): check the right pointer

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1537357 13f79535-47bb-0310-9956-ffa450edef68