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/clouddns.go

@@ -85,11 +85,11 @@ func (h *CloudDNS) Run(ctx context.Context) error {
 		for {
 			select {
 			case <-ctx.Done():
-				log.Infof("Breaking out of CloudDNS update loop: %v", ctx.Err())
+				log.Debugf("Breaking out of CloudDNS update loop for %v: %v", h.zoneNames, ctx.Err())
 				return
 			case <-time.After(1 * time.Minute):
 				if err := h.updateZones(ctx); err != nil && ctx.Err() == nil /* Don't log error if ctx expired. */ {
-					log.Errorf("Failed to update zones: %v", err)
+					log.Errorf("Failed to update zones %v: %v", h.zoneNames, err)
 				}
 			}
 		}