refactor(sync): use task scheduler (#1301)

Signed-off-by: Petu Eusebiu <peusebiu@cisco.com>

errors/errors.go

@@ -53,7 +53,6 @@ var (
 	ErrSyncInvalidUpstreamURL         = errors.New("sync: upstream url not found in sync config")
 	ErrRegistryNoContent              = errors.New("sync: could not find a Content that matches localRepo")
 	ErrSyncReferrerNotFound           = errors.New("sync: couldn't find upstream referrer")
-	ErrSyncReferrer                   = errors.New("sync: failed to get upstream referrer")
 	ErrImageLintAnnotations           = errors.New("routes: lint checks failed")
 	ErrParsingAuthHeader              = errors.New("auth: failed parsing authorization header")
 	ErrBadType                        = errors.New("core: invalid type")
@@ -92,4 +91,7 @@ var (
 	ErrSignConfigDirNotSet            = errors.New("signatures: signature config dir not set")
 	ErrBadManifestDigest              = errors.New("signatures: bad manifest digest")
 	ErrInvalidSignatureType           = errors.New("signatures: invalid signature type")
+	ErrSyncPingRegistry               = errors.New("sync: unable to ping any registry URLs")
+	ErrSyncImageNotSigned             = errors.New("sync: image is not signed")
+	ErrSyncImageFilteredOut           = errors.New("sync: image is filtered out by sync config")
 )