Publish to multiple topics

Enables publishing to various topics according to annotations
on the functions. The function cache is moved up one level so
that it can be shared between the scale from zero code and the
queue proxy.

Unit tests added for new internal methods.

Tested e2e with arkade and the newest queue-worker and RC
gateway image with two queues and an annotation on one of the
functions of com.openfaas.queue. It worked as expected including
with multiple namespace support.

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

gateway/scaling/function_meta.go

@@ -0,0 +1,21 @@
+// Copyright (c) OpenFaaS Author(s). All rights reserved.
+// Licensed under the MIT license. See LICENSE file in the project root for full license information.
+
+package scaling
+
+import (
+	"time"
+)
+
+// FunctionMeta holds the last refresh and any other
+// meta-data needed for caching.
+type FunctionMeta struct {
+	LastRefresh          time.Time
+	ServiceQueryResponse ServiceQueryResponse
+}
+
+// Expired find out whether the cache item has expired with
+// the given expiry duration from when it was stored.
+func (fm *FunctionMeta) Expired(expiry time.Duration) bool {
+	return time.Now().After(fm.LastRefresh.Add(expiry))
+}