zcash_client_backend: Add `MemoryWalletDb::new()`

zcash_client_backend/src/data_api/mem_wallet.rs

@@ -6,7 +6,7 @@ use std::{
     cmp::Ordering,
     collections::{BTreeMap, HashMap, HashSet},
     convert::Infallible,
-    num::NonZeroU32,
+    num::NonZeroU32, hash::Hash,
 };
 use zcash_keys::keys::{AddressGenerationError, DerivationError};
 use zip32::{fingerprint::SeedFingerprint, DiversifierIndex};
@@ -99,6 +99,24 @@ pub struct MemoryWalletDb {
     >,
 }
 
+impl MemoryWalletDb {
+    pub fn new(
+        network: Network,
+        max_checkpoints: usize
+    ) -> Self {
+        Self {
+            network,
+            accounts: BTreeMap::new(),
+            blocks: BTreeMap::new(),
+            tx_idx: HashMap::new(),
+            sapling_spends: BTreeMap::new(),
+            orchard_spends: BTreeMap::new(),
+            sapling_tree: ShardTree::new(MemoryShardStore::empty(), max_checkpoints),
+            orchard_tree: ShardTree::new(MemoryShardStore::empty(), max_checkpoints),
+        }
+    }
+}
+
 #[derive(Debug)]
 pub enum Error {
     AccountUnknown(u32),