Add feature: scale from zero to 1 replicas

This change allows functions to be "idled" or scaled to zero
replicas and then be invoked later on. There is a penalty to
scaling up - the API gateway proxy will block until the function
is ready.

A cache is included to off-set the calls to upstream API to check
on readiness along with unit tests.

Testing via scaling to zero replicas and then invoking function.
On Swarm I observed 3 seconds on an Intel Nuc i5 for scaling back
from zero replicas.

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

gateway/handlers/function_cache_test.go

@@ -0,0 +1,72 @@
+package handlers
+
+import (
+	"testing"
+	"time"
+)
+
+func Test_LastRefreshSet(t *testing.T) {
+	before := time.Now()
+
+	fnName := "echo"
+
+	cache := FunctionCache{
+		Cache:  make(map[string]*FunctionMeta),
+		Expiry: time.Millisecond * 1,
+	}
+
+	if cache.Cache == nil {
+		t.Errorf("Expected cache map to be initialized")
+		t.Fail()
+	}
+
+	cache.Set(fnName, 1)
+
+	if _, exists := cache.Cache[fnName]; !exists {
+		t.Errorf("Expected entry to exist after setting %s", fnName)
+		t.Fail()
+	}
+
+	if cache.Cache[fnName].LastRefresh.Before(before) {
+		t.Errorf("Expected LastRefresh for function to have been after start of test")
+		t.Fail()
+	}
+}
+
+func Test_CacheExpiresIn1MS(t *testing.T) {
+	fnName := "echo"
+
+	cache := FunctionCache{
+		Cache:  make(map[string]*FunctionMeta),
+		Expiry: time.Millisecond * 1,
+	}
+
+	cache.Set(fnName, 1)
+	time.Sleep(time.Millisecond * 2)
+
+	_, hit := cache.Get(fnName)
+
+	wantHit := false
+
+	if hit != wantHit {
+		t.Errorf("hit, want: %v, got %v", wantHit, hit)
+	}
+}
+
+func Test_CacheGivesHitWithLongExpiry(t *testing.T) {
+	fnName := "echo"
+
+	cache := FunctionCache{
+		Cache:  make(map[string]*FunctionMeta),
+		Expiry: time.Millisecond * 500,
+	}
+
+	cache.Set(fnName, 1)
+
+	_, hit := cache.Get(fnName)
+	wantHit := true
+
+	if hit != wantHit {
+		t.Errorf("hit, want: %v, got %v", wantHit, hit)
+	}
+}