wallet, refactor: Convert uint256 to Txid in wallet

Switch all instances of transactions from uint256 to Txid in the
wallet and relevant tests.

src/wallet/interfaces.cpp

@@ -533,7 +533,7 @@ public:
     std::unique_ptr<Handler> handleTransactionChanged(TransactionChangedFn fn) override
     {
         return MakeSignalHandler(m_wallet->NotifyTransactionChanged.connect(
-            [fn](const uint256& txid, ChangeType status) { fn(Txid::FromUint256(txid), status); }));
+            [fn](const Txid& txid, ChangeType status) { fn(txid, status); }));
     }
     std::unique_ptr<Handler> handleWatchOnlyChanged(WatchOnlyChangedFn fn) override
     {