Merge bitcoin/bitcoin#21206: refactor: Make CWalletTx sync state type-safe

d8ee8f3cd3 refactor: Make CWalletTx sync state type-safe (Russell Yanofsky)

Pull request description:

  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.

ACKs for top commit:
  laanwj:
    re-ACK d8ee8f3cd3
  jonatack:
    Code review ACK d8ee8f3cd3

Tree-SHA512: b9f15e9d99dbdbdd3ef7a76764e11f66949f50e6227e284126f209e4cb106af6d55e9a9e8c7d4aa216ddc92c6d5acc6f4aa4746f209bbd77f03831b51a2841c3

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