Make full-scan/sync flow easier to reason about.

[evanlinjin]

Description

#1811 introduces the evicted-at concept. While adding this to TxUpdate, I realized there were some shortcomings in our full-scan & sync flow:

• Chain sources that use TxUpdate to convey updates cannot choose to add transactions without a seen_at timestamp as the TxGraph::apply_update_at logic adds this timestamp for all unanchored txs if the seen_at parameter is specified. Purposefully adding uncanonical txs is useful for wallets that want to know about replaced txs.
• The TxGraph::apply_update_at logic is hard to reason about. TxUpdate::seen_ats already has the seen_at timestamp per tx, but apply_update_at() also takes in a seen_at timestamp`.
• TxUpdate::seen_ats currently forces us to only have one seen-at per tx as it's a map of txid -> seen_at. However, in the future we want to add a first-seen timestamp to TxGraph which is basically the earliest seen-at timestamp introduced so we may want to have multiple seen_ats per tx. The other issue is if we merge TxUpdates, we will loose data. I.e. we can only keep the last_seen or the first_seen.

These problems were exacerbated when introducing evicted-at. In the old design, the chain-source has no concept of sync time (as sync time was introduced after-the-fact when applying the TxUpdate). Therefore the introduced TxUpdate::evicted field was a HashSet<Txid> (no timestamps) and relied on the TxGraph::apply_upate_at logic to introduce the evicted-at timestamp. All this makes the sync logic harder to understand. What happens if the seen_at input is None? What happens if updates were applied out of order? What happens when we merge TxUpdates before applying?

The following changes are made in this PR to simplify the sync/full-scan logic to be easier to understand and robust:

• The sync_time is added to the SyncRequest and FullScanRequest. sync_time is mandatory and is added as an input of builder(). If the std feature is enabled, the builder_now() is available which uses the current timestamp. The chain source is now responsible to add this sync_time timestamp as seen_at for mempool txs. Non-canonical and non-anchored txs can be added without the seen_at timestamp.
• TxUpdate::seen_ats is now a HashSet of (Txid, u64). This allows multiple seen_ats per tx. This is also a much easier to use API as the chain source can just insert into this HashSet without checking previous data.
• TxGraph::apply_update_at is removed as we no longer needs to introduce a fallback seen_at timestamp after-the-fact. TxGraph::apply_update is no longer std-only and the logic of applying updates is greatly simplified.

Additionally, TxUpdate is updated to be #[non_exhaustive] for backwards-compatibility protection.

Notes to the reviewers

This is based on #1837. Merge that first.

These are breaking changes to bdk_core. It needs to be breaking to properly fix all the issues.

As mentioned by @notmandatory, bdk_wallet changes will be added to a new PR once the new bdk_wallet repo is ready. But the PR won't be merged until we're ready for a bdk_wallet 2.0.

Changelog notice

• Change FullScanRequest::builder and SyncRequest::builder methods to depend on feature = "std". This is because requests now have a start_time, instead of specifying a seen_at when applying the update.
• Add FullScanRequest::builder_at and SyncRequest::builder_at methods which are the non-std version of the ..Request::builder methods.
• Change TxUpdate::seen_ats field to be a HashSet of (Txid, u64). This allows a single update to have multiple seen_ats per tx.
• Change TxUpdate to be non-exhaustive.
• Remove apply_update_at as we no longer need to apply with a timestamp after-the-fact.

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 No tests needed as it's more of a refactor.

• I've added docs for the new feature

[LLFourn]

I left a couple of documentation comments but this LGTM.

[ValuedMammal]

Approach ACK

[LLFourn]

ACK 8682487

[nymius]

ACK 8682487

[LagginTimes]

Nit: typo in commit message.

This may, if we introduce new fields to TxUdpate, they can be minor
non-breaking updates.

Otherwise LGTM.

[evanlinjin]

Rebased on #1860 and removed the changes to bdk_wallet.

[notmandatory]

utACK 800f358

Note to other reviewers: bdk_wallet changes will be added to a new PR once the new bdk_wallet repo is ready. But the PR won't be merged until we're ready for a bdk_wallet 2.0.