fix(balancer) delay initialization of balancer objects

When upstreams include targets using hostnames instead of IPs, the
initialization of balancer objects needs to perform DNS resolution, which
involves opening a socket, which is not available in the init_worker phase.

This patch delays balancer initialization to the access phase.

kong/core/handler.lua

@@ -31,6 +31,7 @@ local unpack   = unpack
 
 local router, router_err, router_version
 local server_header = _KONG._NAME.."/".._KONG._VERSION
+local balancer_initialized = false
 
 
 local function get_now()
@@ -85,12 +86,6 @@ return {
       local cluster_events = singletons.cluster_events
 
 
-      -- initialize balancers
-
-
-      balancer.init()
-
-
       -- events dispatcher
 
 
@@ -320,6 +315,14 @@ return {
   },
   access = {
     before = function(ctx)
+
+      -- ensure upstreams are initialized
+
+      if not balancer_initialized then
+        balancer.init()
+        balancer_initialized = true
+      end
+
       -- ensure router is up-to-date
 
       local version, err = singletons.cache:get("router:version", {