Store transaction IDs durably.

The resolution of the project to read at a timestamp without a snapshot has resulted in agreement that we'll need to include transaction IDs as well as timestamps in the new format changes. There are a two different proposals as how best to do that: either to store transaction IDs in the data file, or to store them in the history file with hints in the data file as to what information is included in the history file. In the latter proposal, hints would be stored either in the data file's leaf pages, or in the address cookie in the internal page that references the leaf page.