feat!(chain): implement first_seen tracking

[uvuvuwu]

Description

This PR solves issue #1947 by implementing first_seen tracking.

• Added first_seen field to TxGraph and ChangeSet so that first_seen timestamp can be added when inserting a new seen-at using insert_seen_at.

• first_seen added to TxNode as a way to retrieve the first-seen timestamp for a transaction.

• first_seen added to ChainPosition::Unconfirmed to order unconfirmed transactions by first_seen.

• New tests have been added for the above described functionalities.

Changelog notice

• Add tracking first-seen timestamps of transactions

Checklists

All Submissions:

• I've signed all my commits
• I followed the contribution guidelines
• I ran cargo fmt and cargo clippy before committing

New Features:

• I've added tests for the new feature
• I've added docs for the new feature

[evanlinjin]

Mostly looks good. Some quirks that requires changing in the tests. Also, ChainPosition field order should be changed to make proper use of derive(Ord).

Other things to take note of:

• Documentation comments need full stops!
• Will be nice to include the module changed in commit messages. I.e. feat(chain): signals that we are changing the bdk_chain crate.

[evanlinjin]

LGTM, just documentation suggestions.

[evanlinjin]

ACK 8b17bcf