后端选择策略如果是consistent_hash时，若节点挂掉，无法及时failover到好的节点

[xiewenhui]

后端cls选择节点策略如果是consistent_hash时，
若选到的节点突然挂掉，无法及时fail over到好的节点，会有部分502

[qleein]

@xiewenhui, I agree with you.
In the function "next_consistent_hash_server" in file consistent_hash.lua, line 64.

    local circle = chash.circle
    local st = binary_search(circle, hash_string(hash_key))
    local ed = #circle
    for i = st, ed do  -- TODO: algorithm O(n)
        local idx = circle[i][2]
        if peer_cb(idx, servers[idx]) then
            return servers[idx]
        end
    end

When hash_key maps to the end of circle, as st equals ed after binary_search,
and the node is down, It would never retry another node again. The action is not a hash ring, but a linear array.