the LSN of the prior full version of this node, or NULL_LSN if
there is no prior full version. Used for counting the prior version as
obsolete. If the offset of the LSN is zero, only the file number is
known because we read a version 1 log entry.
getPrevDeltaLsn
long getPrevDeltaLsn()
Returns:
the LSN of the prior delta version of this node, or NULL_LSN if
the prior version is a full version. Used for counting the prior
version as obsolete. If the offset of the LSN is zero, only the file
number is known because we read a version 1 log entry.