Fix issue 4365, multi-threaded svnserve pool memory use after clear.

* subversion/svnserve/svnserve.c
  (struct shared_pool_t): New.
  (attach_shared_pool, release_shared_pool): New.
  (struct serve_thread_t): Replace pool with shared_pool.
  (serve_thread): Use and release shared pool.
  (main): Setup and release shared pool.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1481010 13f79535-47bb-0310-9956-ffa450edef68