consistent hashing for upstreams

[Alexandr Gomoliako]

> The only potential issues I foresee are:

>    1) performance, as this perl will be called for 1000+ requests per
> second, and there are going to be potentially many upstream blocks.
> Maybe Digest::MurmurHash would help with performance instead of MD5
> (it's supposedly 3x faster in Perl than Digest::MD5 while using far less
> state). A native hash ring implementation in C would obviously be far
> more performant.

Couple of microseconds per request isn't something to worry about here.

>    2) a single backup server is problematic, but that can be fixed by
> adding more backups to the upstream blocks I think, or doing an error
> location that hashes again to find a new upstream. Not sure if a server
> being down would cause it to fail inside all upstream blocks it appears
> though, which might mean some very slow responses when a server goes
> offline.

But at least it's simple.

>   3) Perl module is still marked as experimental, which scares me

Don't be scared, it's not really experimental. Build it with
relatively modern perl and you'll be fine.