feat(chain)!: wrap TxGraph txs with Arc

Wrapping transactions as `Arc<Transaction>` allows us to share
transactions cheaply between the chain-source and receiving structures.
Therefore the chain-source can keep already-fetched transactions (save
bandwidth) and have a shared pointer to the transactions (save memory).

This is better than the current way we do things, which is to refer back
to the receiving structures mid-sync.

Documentation for `TxGraph` is also updated.

crates/esplora/tests/async_ext.rs

@@ -66,7 +66,7 @@ pub async fn test_update_tx_graph_without_keychain() -> anyhow::Result<()> {
     for tx in graph_update.full_txs() {
         // Retrieve the calculated fee from `TxGraph`, which will panic if we do not have the
         // floating txouts available from the transactions' previous outputs.
-        let fee = graph_update.calculate_fee(tx.tx).expect("Fee must exist");
+        let fee = graph_update.calculate_fee(&tx.tx).expect("Fee must exist");
 
         // Retrieve the fee in the transaction data from `bitcoind`.
         let tx_fee = env

crates/esplora/tests/blocking_ext.rs

@@ -80,7 +80,7 @@ pub fn test_update_tx_graph_without_keychain() -> anyhow::Result<()> {
     for tx in graph_update.full_txs() {
         // Retrieve the calculated fee from `TxGraph`, which will panic if we do not have the
         // floating txouts available from the transactions' previous outputs.
-        let fee = graph_update.calculate_fee(tx.tx).expect("Fee must exist");
+        let fee = graph_update.calculate_fee(&tx.tx).expect("Fee must exist");
 
         // Retrieve the fee in the transaction data from `bitcoind`.
         let tx_fee = env