worker: Improve log message
worker: Improve log message

Issue #3309
The online cloning process needs to read from both source and
destination. Changed to log message to make it clear.

dcadevil/vitess,enisoc/vitess,HubSpot/vitess,mahak/vitess,tinyspeck/vitess,mahak/vitess,enisoc/vitess,davygeek/vitess,tirsen/vitess,tinyspeck/vitess,sougou/vitess,vitessio/vitess,HubSpot/vitess,mahak/vitess,mattharden/vitess,tirsen/vitess,dcadevil/vitess,tirsen/vitess,tinyspeck/vitess,mattharden/vitess,enisoc/vitess,mattharden/vitess,sougou/vitess,vitessio/vitess,sougou/vitess,alainjobart/vitess,dcadevil/vitess,mahak/vitess,sougou/vitess,HubSpot/vitess,sougou/vitess,vitessio/vitess,vitessio/vitess,davygeek/vitess,HubSpot/vitess,HubSpot/vitess,mahak/vitess,tirsen/vitess,sougou/vitess,mahak/vitess,enisoc/vitess,davygeek/vitess,tirsen/vitess,tinyspeck/vitess,mattharden/vitess,alainjobart/vitess,sougou/vitess,alainjobart/vitess,mahak/vitess,dcadevil/vitess,mattharden/vitess,dcadevil/vitess,tinyspeck/vitess,tirsen/vitess,mattharden/vitess,enisoc/vitess,vitessio/vitess,mattharden/vitess,tirsen/vitess,HubSpot/vitess,mattharden/vitess,alainjobart/vitess,davygeek/vitess,mattharden/vitess,vitessio/vitess,alainjobart/vitess,tinyspeck/vitess,vitessio/vitess,tinyspeck/vitess,HubSpot/vitess,dcadevil/vitess,davygeek/vitess,davygeek/vitess,enisoc/vitess,davygeek/vitess,tirsen/vitess,alainjobart/vitess,alainjobart/vitess,HubSpot/vitess,mattharden/vitess,vitessio/vitess,tirsen/vitess,alainjobart/vitess,mahak/vitess,dcadevil/vitess,enisoc/vitess,sougou/vitess