Build an red-black tree to store upstreams added by the directive "upstream"

If variables are provided in "proxy_pass", each request would trigger a search in the list of upstream. The search can be really slow if the number of upstreams is relatively large, which causes a degradation in QPS. To solve this problem, I build an red-black tree to store upstreams added by the directive "upstream", so as to make the search faster and improve the QPS. 
An option called "--without-http-upstream-rbtree" is provided to disable this feature.