Provide a pool for open repository instances.  Using the new
object pool API, this is straightforward.

* subversion/include/private/svn_repos_private.h
  (svn_repos__repos_pool_t,
   svn_repos__repos_pool_create,
   svn_repos__repos_pool_get): declare new repository pool API

* subversion/libsvn_repos/repos_pool.c
  (): new file
  (svn_repos__repos_pool_t): define the new pool type
  (construct_key): new utility function
  (svn_repos__repos_pool_create,
   svn_repos__repos_pool_get): implement the new API


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1540595 13f79535-47bb-0310-9956-ffa450edef68