Make sure pre-serialized values are deleted after flush

[badrishc]

• When we PostCopyUpdate an object, we replace the old image in the log with its serialized representation so that we have a consistent old version of the object. This PR optimizes this case to occur only during a snapshot checkpoint, because the (v) version needs to be captured only in that case.
• The disk flush thread (either snapshot or head page flush) can pick up this serialized data for version (v) to write to disk.
• We currently leave the serialized data intact in memory which causes heap to grow with repeated checkpoints. In this PR, we scan the log after the checkpoint to replace it with a null, in order to save on extra memory use.
• Scan was extended to support scanning sealed records, as the serialized data is attached to the sealed records.
• A subsequent snapshot can write Null to storage for this object, as we know the object is subsumed by a newer version thanks to the PCU.
• Recovering from a checkpoint will reconstitute a null object, which is fine because a newer version of the object is known to exist.
• This means we may now have null objects on the log - this is fine because Scan will always check for new version. We added support for a null object type in the serialized representation.