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>

example-crates/wallet_electrum/src/main.rs

@@ -35,7 +35,7 @@ fn main() -> Result<(), Box<dyn std::error::Error>> {
     print!("Syncing...");
     let client = electrum_client::Client::new("ssl://electrum.blockstream.info:60002")?;
 
-    let local_chain = wallet.checkpoints();
+    let prev_tip = wallet.latest_checkpoint();
     let keychain_spks = wallet
         .spks_of_all_keychains()
         .into_iter()
@@ -52,8 +52,7 @@ fn main() -> Result<(), Box<dyn std::error::Error>> {
         })
         .collect();
 
-    let electrum_update =
-        client.scan(local_chain, keychain_spks, None, None, STOP_GAP, BATCH_SIZE)?;
+    let electrum_update = client.scan(prev_tip, keychain_spks, None, None, STOP_GAP, BATCH_SIZE)?;
 
     println!();