long value indicated serial number of current count of changes
to hasNeededUndonePiece.
A method interesting in tracking changes can compare this with a locally stored
value to determine if the hasNeededUndonePiece status has changed since the last check.
This is called periodically by the peer control scheduler.
It should not normally be called by other methods.
It will update the global availability if neccesary
and then update the derived information
