Upstream hash: speedup consistent hash init.

Upstream hash: speedup consistent hash init.

Repeatedly calling ngx_http_upstream_add_chash_point() to create
the points array in sorted order, is O(n^2) to the total weight.
This can cause nginx startup and reconfigure to be substantially
delayed.  For example, when total weight is 1000, startup takes
5s on a modern laptop.

Replace this with a linear insertion followed by QuickSort and
duplicates removal.  Startup for total weight of 1000 reduces to 40ms.

Based on a patch by Wai Keen Woon.