Simply APR memory pool handling in svnserve.  Allocate all connection_t
objects from the application's main pool instead of given them independent
root pools.  Since we (potentially) release connections in background
threads, the applications's root pool must be thread-safe.

* subversion/svnserve/svnserve.c
  (accept_connection): Allocate the new connection in a sub-pool of the
                       pool passed in - just following our standard pool
                       usage pattern.
  (close_connection): Pool destruction now uses standard code as well.
  (sub_main): We don't need the socket_pools anymore.  Update call to
              above sub-function.
  (main): Make sure the application's root pool is thread-safe.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1581906 13f79535-47bb-0310-9956-ffa450edef68