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/bench/coin_selection.cpp

@@ -18,7 +18,7 @@ static void addCoin(const CAmount& nValue, const CWallet& wallet, std::vector<st
     tx.nLockTime = nextLockTime++; // so all transactions get different hashes
     tx.vout.resize(1);
     tx.vout[0].nValue = nValue;
-    wtxs.push_back(std::make_unique<CWalletTx>(MakeTransactionRef(std::move(tx))));
+    wtxs.push_back(std::make_unique<CWalletTx>(MakeTransactionRef(std::move(tx)), TxStateInactive{}));
 }
 
 // Simple benchmark for wallet coin selection. Note that it maybe be necessary