Further improve svnserve's responsiveness to large numbers of small
requests in threaded mode: Use a thread pool.

* subversion/svnserve/svnserve.c
  (THREADPOOL_MIN_SIZE,
   THREADPOOL_MAX_SIZE,
   THREADPOOL_THREAD_IDLE_LIMIT): new tuning knobs controlling the use
                                  of the thread pool
  (main): create thread pool; use it instead of indiviual, detached threads

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1523476 13f79535-47bb-0310-9956-ffa450edef68