refactor balancer into more testable and extensible interface

rootfs/etc/nginx/lua/balancer/round_robin.lua

@@ -0,0 +1,20 @@
+local balancer_resty = require("balancer.resty")
+local resty_roundrobin = require("resty.roundrobin")
+local util = require("util")
+
+local _M = balancer_resty:new({ factory = resty_roundrobin, name = "round_robin" })
+
+function _M.new(self, backend)
+  local nodes = util.get_nodes(backend.endpoints)
+  local o = { instance = self.factory:new(nodes) }
+  setmetatable(o, self)
+  self.__index = self
+  return o
+end
+
+function _M.balance(self)
+  local endpoint_string = self.instance:find()
+  return util.split_pair(endpoint_string, ":")
+end
+
+return _M