[chain] It should be easy to get the transaction in canonical order

[LLFourn]

So as ordered by most recent to oldest vise versa

What do we mean by this?

1. We could mean that an tx that was confirmed earlier. But what if it was broadcast earlier? Do we want transactions switching order.
2. We could introduce the concept of first_seen which stores the first time you see the tx. Ordered by first_seen and tiebroken by confirmation height should give a nice consistent list of transactions that doesn't switch order too much. Note this wouldn't change ChangeSet behavior at all since we already have last_seen.
3. Do transactions that are non-canonical belong in a list? e.g. transactions that were RBF'd.

[notmandatory]

Can we move this to a post-1.0 milestone? it looks like a feature that can be added with a new function without breaking any of the existing apis.