Iterate peers as linked list instead of array

[quezacoatl]

Accessing peers as an array has been deprecated since nginx 1.9.0 (changeset 6100 - http://hg.nginx.org/nginx/rev/c44459611d91) and does not work when using the zone directive in upstream block. This commit changes iteration to use peer->next and iterate as a linked list.

This PR should solve #31 and #61

[quezacoatl]

I got this reply when I asked regarding this in nginx-devel mailing list:

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The ngx_lua_upstream module is not adapted to work with peers stored
in memory as a linked list, thus it cannot be used with upstream zone
that copies peers from configuration to shared memory one by one, thus
the peers' layout is effectively changed from an array to a linked list.
The problem you observe is that lua iterates peers by index as an array.

See this change as a guidance:
http://hg.nginx.org/nginx/rev/c44459611d91