Replace local code in svnserve with the new svn_root_pools__* API.
Use separate pools for the many small socket objects and the few,
larger connections.

The number of connections to a svnserve instance now increases its
memory consumption by less than 100kB per additional connection.
Tested with 20000 concurrent connections.

* subversion/svnserve/svnserve.c
  (connection_pools,
   connection_pools_mutex): drop global variables
  (initialize_connection_pools,
   acquire_connection_pool_internal,
   acquire_connection_pool,
   release_connection_pool): drop obsolete functions
  (shared_pool_t,
   attach_shared_pool,
   release_shared_pool): add the root pools container so we can return
                         the socket pool there
  (connection_pools,
   serve_thread): use a separate root pool container for connections
  (main): init the new root pool containers;
          use a socket pool instead of connection pool

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1527142 13f79535-47bb-0310-9956-ffa450edef68