Auto merge of #3106 - braddmiller:3046-mergetoaddress-concurrent, r=str4d

Add Note Locking to z_mergetoaddress

Adds note locking to `z_mergetoaddress` allowing it to be invoked multiple times before previous `z_mergetoaddress` operations have finished.

Reference issue [#3046](https://github.com/zcash/zcash/issues/3046)

Co-authored-by: Eirik Ogilvie-Wigley <eirik@z.cash>

src/wallet/wallet.cpp

@@ -3443,6 +3443,42 @@ void CWallet::ListLockedCoins(std::vector<COutPoint>& vOutpts)
     }
 }
 
+
+// Note Locking Operations
+
+void CWallet::LockNote(JSOutPoint& output)
+{
+    AssertLockHeld(cs_wallet); // setLockedNotes
+    setLockedNotes.insert(output);
+}
+
+void CWallet::UnlockNote(JSOutPoint& output)
+{
+    AssertLockHeld(cs_wallet); // setLockedNotes
+    setLockedNotes.erase(output);
+}
+
+void CWallet::UnlockAllNotes()
+{
+    AssertLockHeld(cs_wallet); // setLockedNotes
+    setLockedNotes.clear();
+}
+
+bool CWallet::IsLockedNote(uint256 hash, size_t js, uint8_t n) const
+{
+    AssertLockHeld(cs_wallet); // setLockedNotes
+    JSOutPoint outpt(hash, js, n);
+
+    return (setLockedNotes.count(outpt) > 0);
+}
+
+std::vector<JSOutPoint> CWallet::ListLockedNotes()
+{
+    AssertLockHeld(cs_wallet); // setLockedNotes
+    std::vector<JSOutPoint> vOutpts(setLockedNotes.begin(), setLockedNotes.end());
+    return vOutpts;
+}
+
 /** @} */ // end of Actions
 
 class CAffectedKeysVisitor : public boost::static_visitor<void> {
@@ -3731,6 +3767,11 @@ void CWallet::GetFilteredNotes(
             if (ignoreUnspendable && !HaveSpendingKey(pa)) {
                 continue;
             }
+            
+            // skip locked notes
+            if (IsLockedNote(jsop.hash, jsop.js, jsop.n)) {
+                continue;
+            }
 
             int i = jsop.js; // Index into CTransaction.vjoinsplit
             int j = jsop.n; // Index into JSDescription.ciphertexts