streaming: double the cache TTL

10 minutes is the default blocking query timeout. Using the same value results in us hitting
the expired cache entry bug frequently. By extending this TTL we at least mitigate the problem.

The underlying bug still needs to be fixed.

agent/cache-types/streaming_health_services.go

@@ -39,7 +39,7 @@ type StreamingHealthServices struct {
 // so using a shorter TTL ensures the cache entry expires sooner.
 func (c *StreamingHealthServices) RegisterOptions() cache.RegisterOptions {
 	opts := c.RegisterOptionsBlockingRefresh.RegisterOptions()
-	opts.LastGetTTL = 10 * time.Minute
+	opts.LastGetTTL = 20 * time.Minute
 	return opts
 }