Magma size based history retention breaks on upgrade to 7.6.x

In 7.6.0, to support collection level history, we added a stat called as HistoryLogicalSize for every sstable. This tells us the amount of data in a sstable that is part of the history (patch: https://review.couchbase.org/c/magma/+/192225)

We also changed our history retention window logic to retain sstables based on this new stat. Earlier we could consider an entire sstable's size as being its history size. (patch: https://review.couchbase.org/c/magma/+/194938/5/lsm/lsm_tree.cc#2107)

However, this stat is not available in 7.2.x. And defaults to a very large garbage value (though deterministic) 969246306729132037. This breaks history retention. It will cause the user to lose all history on upgrade.

We have to also enhance our existing upgrade tests (both unit test level as well as QE's) to test upgrades from history enabled data and assert some stats.