Refactor scaling to use existing code

Existing code has been used for scaling up and querying replicas.
This meant the new code was deleted and there is less duplication
now.

The cache store a whole query response rather than just the
available replica count and the tests were updated. This has been
tested with Docker swarm and the image:
 openfaas/gateway:scale-17-07-2018

This feature now needs the env-var of scale_from_zero to be enabled
in order to turn on the scaling behaviour.

Signed-off-by: Alex Ellis (VMware) <alexellis2@gmail.com>

gateway/handlers/function_cache.go

@@ -11,8 +11,8 @@ import (
 // FunctionMeta holds the last refresh and any other
 // meta-data needed for caching.
 type FunctionMeta struct {
-	LastRefresh time.Time
-	Replicas    uint64
+	LastRefresh          time.Time
+	ServiceQueryResponse ServiceQueryResponse
 }
 
 // Expired find out whether the cache item has expired with
@@ -29,7 +29,7 @@ type FunctionCache struct {
 }
 
 // Set replica count for functionName
-func (fc *FunctionCache) Set(functionName string, replicas uint64) {
+func (fc *FunctionCache) Set(functionName string, serviceQueryResponse ServiceQueryResponse) {
 	fc.Sync.Lock()
 
 	if _, exists := fc.Cache[functionName]; !exists {
@@ -38,19 +38,22 @@ func (fc *FunctionCache) Set(functionName string, replicas uint64) {
 
 	entry := fc.Cache[functionName]
 	entry.LastRefresh = time.Now()
-	entry.Replicas = replicas
+	entry.ServiceQueryResponse = serviceQueryResponse
 
 	fc.Sync.Unlock()
 }
 
 // Get replica count for functionName
-func (fc *FunctionCache) Get(functionName string) (uint64, bool) {
-	replicas := uint64(0)
+func (fc *FunctionCache) Get(functionName string) (ServiceQueryResponse, bool) {
+	replicas := ServiceQueryResponse{
+		AvailableReplicas: 0,
+	}
+
 	hit := false
 	fc.Sync.Lock()
 
 	if val, exists := fc.Cache[functionName]; exists {
-		replicas = val.Replicas
+		replicas = val.ServiceQueryResponse
 		hit = !val.Expired(fc.Expiry)
 	}