Added simplified hostcache implementation for rest client

ably/rest_client.go

@@ -931,3 +931,29 @@ func decodeResp(resp *http.Response, out interface{}) error {
 
 	return decode(typ, bytes.NewReader(b), out)
 }
+
+// hostCache caches a successful fallback host for 10 minutes.
+// Only used by REST client while making requests RSC15f
+type hostCache struct {
+	duration time.Duration
+
+	sync.RWMutex
+	deadline time.Time
+	host     string
+}
+
+func (c *hostCache) put(host string) {
+	c.Lock()
+	defer c.Unlock()
+	c.host = host
+	c.deadline = time.Now().Add(c.duration)
+}
+
+func (c *hostCache) get() string {
+	c.RLock()
+	defer c.RUnlock()
+	if ablyutil.Empty(c.host) || time.Until(c.deadline) <= 0 {
+		return ""
+	}
+	return c.host
+}