Only "unpause" the producer stream if it may progress

In the current implementation the DcpProducer use the following logic:

unpause()

if may progress

    return

pause() 

By unconditionally unpause before checking if any progress may be done we loose the information on how long we actually pause. This should be changed to:

if may progress

   unpause

   return

if not already paused

   pause 

Now we can keep track of the duration we're continuously paused