First shot at a configuration pool.  This data structure allows
to cache parsed svn_config_t objects to be cached and shared
between threads.

We will use this code in svnserve's upcoming repository cache.

Code is untested and tests are yet to be written. 

* develop/trunk/subversion/include/private/svn_subr_private.h
  (svn_config_pool__t,
   svn_config_pool__create,
   svn_config_pool__get): declare new API

* develop/trunk/subversion/libsvn_subr/config_pool.c
  (): new file implementing the new API

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1532239 13f79535-47bb-0310-9956-ffa450edef68