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/chain/tests/test_indexed_tx_graph.rs

@@ -109,8 +109,8 @@ fn test_list_owned_txouts() {
     // Create Local chains
 
     let local_chain = (0..150)
-        .map(|i| (i as u32, h!("random")))
-        .collect::<BTreeMap<u32, BlockHash>>();
+        .map(|i| (i as u32, Some(h!("random"))))
+        .collect::<BTreeMap<u32, Option<BlockHash>>>();
     let local_chain = LocalChain::from(local_chain);
 
     // Initiate IndexedTxGraph
@@ -212,9 +212,10 @@ fn test_list_owned_txouts() {
             (
                 *tx,
                 local_chain
-                    .blocks()
+                    .heights()
                     .get(&height)
-                    .map(|&hash| BlockId { height, hash })
+                    .cloned()
+                    .map(|hash| BlockId { height, hash })
                     .map(|anchor_block| ConfirmationHeightAnchor {
                         anchor_block,
                         confirmation_height: anchor_block.height,
@@ -231,10 +232,10 @@ fn test_list_owned_txouts() {
         |height: u32,
          graph: &IndexedTxGraph<ConfirmationHeightAnchor, KeychainTxOutIndex<String>>| {
             let chain_tip = local_chain
-                .blocks()
+                .heights()
                 .get(&height)
                 .map(|&hash| BlockId { height, hash })
-                .expect("block must exist");
+                .unwrap_or_else(|| panic!("block must exist at {}", height));
             let txouts = graph
                 .graph()
                 .filter_chain_txouts(