Fixes #1354

Description

Built on top of both #1369 and #1373, we simplify the EsploraExt API by removing the update_local_chain method and having full_scan and sync update the local chain in the same call. The full_scan and sync methods now takes in an additional input (local_tip) which provides us with the view of the LocalChain before the update. These methods now return structs FullScanUpdate and SyncUpdate.

The examples are updated to use this new API. TxGraph::missing_heights and tx_graph::ChangeSet::missing_heights_from are no longer needed, therefore they are removed.

Additionally, we used this opportunity to simplify the logic which updates LocalChain. We got rid of the local_chain::Update struct (which contained the update CheckPoint tip and a bool which signaled whether we want to introduce blocks below point of agreement). It turns out we can use something like CheckPoint::insert so the chain source can craft an update based on the old tip. This way, we can make better use of merge_chains' optimization that compares the Arc pointers of the local and update chain (before we were crafting the update chain NOT based on top of the previous local chain). With this, we no longer need the Update::introduce_older_block field since the logic will naturally break when we reach a matching Arc pointer.

Notes to the reviewers

• Obtaining the LocalChain's update now happens within EsploraExt::full_scan and EsploraExt::sync. Creating the LocalChain update is now split into two methods (fetch_latest_blocks and chain_update) that are called before and after fetching transactions and anchors.
• We need to duplicate code for bdk_esplora. One for blocking and one for async.

Changelog notice

• Changed EsploraExt API so that sync only requires one round of fetching data. The local_chain_update method is removed and the local_tip parameter is added to the full_scan and sync methods.
• Removed TxGraph::missing_heights and tx_graph::ChangeSet::missing_heights_from methods.
• Introduced CheckPoint::insert which allows convenient checkpoint-insertion. This is intended for use by chain-sources when crafting an update.
• Refactored merge_chains to also return the resultant CheckPoint tip.
• Optimized the update LocalChain logic - use the update CheckPoint as the new CheckPoint tip when possible.

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