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/util/overloaded.h

@@ -0,0 +1,22 @@
+// 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.
+
+#ifndef BITCOIN_UTIL_OVERLOADED_H
+#define BITCOIN_UTIL_OVERLOADED_H
+
+namespace util {
+//! Overloaded helper for std::visit. This helper and std::visit in general are
+//! useful to write code that switches on a variant type. Unlike if/else-if and
+//! switch/case statements, std::visit will trigger compile errors if there are
+//! unhandled cases.
+//!
+//! Implementation comes from and example usage can be found at
+//! https://en.cppreference.com/w/cpp/utility/variant/visit#Example
+template<class... Ts> struct Overloaded : Ts... { using Ts::operator()...; };
+
+//! Explicit deduction guide (not needed as of C++20)
+template<class... Ts> Overloaded(Ts...) -> Overloaded<Ts...>;
+} // namespace util
+
+#endif // BITCOIN_UTIL_OVERLOADED_H