Fix following compilation warning(s)

warning: 'Producer::flush' hides overloaded virtual function
[-Woverloaded-virtual]

Change-Id: I9d7861993fb23908ac79662c55c59a409a383eac
Reviewed-on: http://review.couchbase.org/32347
Reviewed-by: David Liao <david.liao@couchbase.com>
Tested-by: abhinav dangeti <abhinav@couchbase.com>