Use cancelable contexts for cloud provider plugin refreshes (#4226)

This commit uses a cancelable context to spawn goroutines that refresh
records from a cloud DNS provider. The Caddy shutdown routine uses the
returned cancel function to terminate existing goroutines when a USR1
reload signal is received.

Signed-off-by: Matt Kulka <mkulka@parchment.com>

plugin/clouddns/setup.go

@@ -78,7 +78,7 @@ func setup(c *caddy.Controller) error {
 			}
 		}
 
-		ctx := context.Background()
+		ctx, cancel := context.WithCancel(context.Background())
 		client, err := f(ctx, opt)
 		if err != nil {
 			return err
@@ -98,7 +98,7 @@ func setup(c *caddy.Controller) error {
 			h.Next = next
 			return h
 		})
-		c.OnShutdown(func() error { ctx.Done(); return nil })
+		c.OnShutdown(func() error { cancel(); return nil })
 	}
 
 	return nil