Fix stalls during checkout/update over http(s): when one connection is slower
than the other connections, up to REQUEST_COUNT_TO_RESUME requests can queue
up on that connection. When that happens, ra_serf will completely stop sending
requests until the slower connection has catched up.

The solution is to assign requests to the connection with the least amount of
pending requests.

Note: requires serf trunk@2421.

* subversion/libsvn_ra_serf/update.c
  (get_best_connection): Add a better method to assign requests to connections,
      based on the minimum number of pending requests on a connections. Keep the
      current method of simply cycling connections for serf<1.4.0.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1621596 13f79535-47bb-0310-9956-ffa450edef68