refactor: Make CWalletTx sync state type-safe

Current CWalletTx state representation makes it possible to set
inconsistent states that won't be handled correctly by wallet sync code
or serialized & deserialized back into the same form.

For example, it is possible to call setConflicted without setting a
conflicting block hash, or setConfirmed with no transaction index. And
it's possible update individual m_confirm and fInMempool data fields
without setting an overall consistent state that can be serialized and
handled correctly.

Fix this without changing behavior by using std::variant, instead of an
enum and collection of fields, to represent sync state, so state
tracking code is safer and more legible.

This is a first step to fixing state tracking bugs
https://github.com/bitcoin-core/bitcoin-devwiki/wiki/Wallet-Transaction-Conflict-Tracking,
by adding an extra margin of safety that can prevent new bugs from being
introduced as existing bugs are fixed.

src/wallet/test/wallet_transaction_tests.cpp

@@ -0,0 +1,24 @@
+// Copyright (c) 2021 The Bitcoin Core developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+#include <wallet/transaction.h>
+
+#include <wallet/test/wallet_test_fixture.h>
+
+#include <boost/test/unit_test.hpp>
+
+BOOST_FIXTURE_TEST_SUITE(wallet_transaction_tests, WalletTestingSetup)
+
+BOOST_AUTO_TEST_CASE(roundtrip)
+{
+    for (uint8_t hash = 0; hash < 5; ++hash) {
+        for (int index = -2; index < 3; ++index) {
+            TxState state = TxStateInterpretSerialized(TxStateUnrecognized{uint256{hash}, index});
+            BOOST_CHECK_EQUAL(TxStateSerializedBlockHash(state), uint256{hash});
+            BOOST_CHECK_EQUAL(TxStateSerializedIndex(state), index);
+        }
+    }
+}
+
+BOOST_AUTO_TEST_SUITE_END()