Use batch decryption in wallet scanning.

This modifies wallet scanning to perform per-block batched
decryption. It also alters the structure of the `ScanningKey`
trait to correctly include internal (change) keys in the scan
process.


Extracted from: 73314dc682

src/batch.rs

@@ -47,6 +47,10 @@ fn batch_note_decryption<D: BatchDomain, Output: ShieldedOutput<D, CS>, F, FR, c
 where
     F: Fn(&D, &D::IncomingViewingKey, &EphemeralKeyBytes, &Output, &D::SymmetricKey) -> Option<FR>,
 {
+    if ivks.is_empty() {
+        return (0..outputs.len()).map(|_| None).collect();
+    };
+
     // Fetch the ephemeral keys for each output and batch-parse them.
     let ephemeral_keys = D::batch_epk(outputs.iter().map(|(_, output)| output.ephemeral_key()));