Merge of r4622, r4623: balancing changes.

Merge of r4622, r4623: balancing changes.

*) Upstream: smooth weighted round-robin balancing.

   For edge case weights like { 5, 1, 1 } we now produce { a, a, b, a, c, a, a }
   sequence instead of { c, b, a, a, a, a, a } produced previously.

   Algorithm is as follows: on each peer selection we increase current_weight
   of each eligible peer by its weight, select peer with greatest current_weight
   and reduce its current_weight by total number of weight points distributed
   among peers.

   In case of { 5, 1, 1 } weights this gives the following sequence of
   current_weight's:

        a  b  c
        0  0  0  (initial state)

        5  1  1  (a selected)
       -2  1  1

        3  2  2  (a selected)
       -4  2  2

        1  3  3  (b selected)
        1 -4  3

        6 -3  4  (a selected)
       -1 -3  4

        4 -2  5  (c selected)
        4 -2 -2

        9 -1 -1  (a selected)
        2 -1 -1

        7  0  0  (a selected)
        0  0  0

   To preserve weight reduction in case of failures the effective_weight
   variable was introduced, which usually matches peer's weight, but is
   reduced temporarily on peer failures.

   This change also fixes loop with backup servers and proxy_next_upstream
   http_404 (ticket #47), and skipping alive upstreams in some cases if there
   are multiple dead ones (ticket #64).

*) Upstream: fixed ip_hash rebalancing with the "down" flag.

   Due to weight being set to 0 for down peers, order of peers after sorting
   wasn't the same as without the "down" flag (with down peers at the end),
   resulting in client rebalancing for clients on other servers.  The only
   rebalancing which should happen after adding "down" to a server is one
   for clients on the server.

   The problem was introduced in r1377 (which fixed endless loop by setting
   weight to 0 for down servers).  The loop is no longer possible with new
   smooth algorithm, so preserving original weight is safe.