In-progress transactions may become visible when transactions are checkpointed

When a transaction is created, the current in-progress list is attached to the transaction object. This list is used to filter out in-progress transactions during read.

The transaction object expects this list to be sorted numerically since it does a binary search for filtering out the in-progress transactions.

When checkpointing feature was added in TEPHRA-96, checkpoints were also added as part of the in-progress list since data writes from checkpoints also have to be filtered out during reads. However adding checkpoints broke the sort order. This leads to binary search not working as expected to filter out in-progress transactions.