Accurately Track Checkpoint Memory Usage

For the checkpointQueue (which is a member of the checkpointManager we record the size of the items added to the queue (which is currently implemented as an stl:list) however does not include the size of the queue overhead (which in the case of a list is relatively large for each element).

To get a better measure of checkpoint memory usage we want to include the overhead
of the checkpointQueue data structure.