support cluster based on consistent hash

[yanxurui]

Hello, dear @agentzh

I add support of consistent hash using resty/chash. We have been using this in production for months.

I have no confidence you will merge this but still I post it here in case someone else needs it.

Thanks.

[yanxurui]

todo: I forgot to remove it. This bug is not exposed because lower case doesn't take effect.

[agentzh]

@yanxurui It's better be a separate Lua library since I don't want lua-resty-memcached to depend on lua-resty-balancer.

[yanxurui]

@agentzh No problem. I will maintain this feature branch in my repo.

[vitobotta]

@yanxurui Hi! Did you create a new library for this? Your fork seems out of date now. Thanks!

[vitobotta]

Actually the main library doesn't seem to have changed much since then. I will try your fork :)

[yanxurui]

@vitobotta I have not touched this code since I left my last employer. Feel free to update my repo if needed.