chore(bdk-electrum): use new batch_transaction_get_merkle API

- removes the now unused `serde_json` dependency from `bdk_electrum`.
- update `batch_fetch_anchors` to use `batch_transaction_get_merkle`
  method instead of manually creating the batch call.

Author: oleonardolima

crates/electrum/Cargo.toml

@@ -14,8 +14,7 @@ workspace = true
 
 [dependencies]
 bdk_core = { path = "../core", version = "0.6.0" }
-electrum-client = { version = "0.23.1", features = [ "proxy" ], default-features = false }
-serde_json = "1.0"
+electrum-client = { version = "0.24.0", features = [ "proxy" ], default-features = false }
 
 [dev-dependencies]
 bdk_testenv = { path = "../testenv" }

crates/electrum/src/bdk_electrum_client.rs

@@ -551,23 +551,12 @@ impl<E: ElectrumApi> BdkElectrumClient<E> {
             }
         }
 
-        // Batch all get_merkle calls.
-        let mut batch = electrum_client::Batch::default();
-        for &(txid, height, _) in &to_fetch {
-            batch.raw(
-                "blockchain.transaction.get_merkle".into(),
-                vec![
-                    electrum_client::Param::String(format!("{txid:x}")),
-                    electrum_client::Param::Usize(height),
-                ],
-            );
-        }
-        let resps = self.inner.batch_call(&batch)?;
+        // Fetch merkle proofs.
+        let txids_and_heights = to_fetch.iter().map(|&(txid, height, _)| (txid, height));
+        let proofs = self.inner.batch_transaction_get_merkle(txids_and_heights)?;
 
         // Validate each proof, retrying once for each stale header.
-        for ((txid, height, hash), resp) in to_fetch.into_iter().zip(resps.into_iter()) {
-            let proof: electrum_client::GetMerkleRes = serde_json::from_value(resp)?;
-
+        for ((txid, height, hash), proof) in to_fetch.into_iter().zip(proofs.into_iter()) {
             let mut header = {
                 let cache = self.block_header_cache.lock().unwrap();
                 cache