Avoid building PartitionUpdate in toString()

In AbstractBTreePartition.toString() (which PartitionUpdate extends), we iterate over the rows in the partition. This triggers maybeBuild() in the PartitionUpdate. If the PartitionUpdate gets updated after the toString() call, it will result in an IllegalStateException with the message "An update should not be written again once it has been read".

As a result, logging or using a debugger can trigger spurious errors, which makes debugging difficult or impossible.