Implement linked-list LocalChain and update chain-src crates/examples

This commit changes the `LocalChain` implementation to have blocks
stored as a linked-list. This allows the data-src thread to hold a
shared ref to a single checkpoint and have access to the whole history
of checkpoints without cloning or keeping a lock on `LocalChain`.

The APIs of `bdk::Wallet`, `esplora` and `electrum` are also updated to
reflect these changes. Note that the `esplora` crate is rewritten to
anchor txs in the confirmation block (using the esplora API's tx status
block_hash). This guarantees 100% consistency between anchor blocks and
their transactions (instead of anchoring txs to the latest tip).
`ExploraExt` now has separate methods for updating the `TxGraph` and
`LocalChain`.

A new method `TxGraph::missing_blocks` is introduced for finding
"floating anchors" of a `TxGraph` update (given a chain).

Additional changes:

* `test_local_chain.rs` is refactored to make test cases easier to
  write. Additional tests are also added.
* Examples are updated.
* Fix `tempfile` dev dependency of `bdk_file_store` to work with MSRV

Co-authored-by: LLFourn <lloyd.fourn@gmail.com>

crates/esplora/src/lib.rs

@@ -14,16 +14,22 @@ mod async_ext;
 #[cfg(feature = "async")]
 pub use async_ext::*;
 
-pub(crate) fn map_confirmation_time_anchor(
-    tx_status: &TxStatus,
-    tip_at_start: BlockId,
-) -> Option<ConfirmationTimeAnchor> {
-    match (tx_status.block_time, tx_status.block_height) {
-        (Some(confirmation_time), Some(confirmation_height)) => Some(ConfirmationTimeAnchor {
-            anchor_block: tip_at_start,
-            confirmation_height,
-            confirmation_time,
-        }),
-        _ => None,
+const ASSUME_FINAL_DEPTH: u32 = 15;
+
+fn anchor_from_status(status: &TxStatus) -> Option<ConfirmationTimeAnchor> {
+    if let TxStatus {
+        block_height: Some(height),
+        block_hash: Some(hash),
+        block_time: Some(time),
+        ..
+    } = status.clone()
+    {
+        Some(ConfirmationTimeAnchor {
+            anchor_block: BlockId { height, hash },
+            confirmation_height: height,
+            confirmation_time: time,
+        })
+    } else {
+        None
     }
 }