MINOR: server: add the srv_queue() sample fetch method

MINOR: server: add the srv_queue() sample fetch method

srv_queue([<backend>/]<server>) : integer
  Returns an integer value corresponding to the number of connections currently
  pending in the designated server's queue. If <backend> is omitted, then the
  server is looked up in the current backend. It can sometimes be used together
  with the "use-server" directive to force to use a known faster server when it
  is not much loaded. See also the "srv_conn", "avg_queue" and "queue" sample
  fetch methods.