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/walletdb.cpp

@@ -987,7 +987,7 @@ DBErrors WalletBatch::FindWalletTx(std::vector<uint256>& vTxHash, std::list<CWal
                 uint256 hash;
                 ssKey >> hash;
                 vTxHash.push_back(hash);
-                vWtx.emplace_back(nullptr /* tx */);
+                vWtx.emplace_back(/*tx=*/nullptr, TxStateInactive{});
                 ssValue >> vWtx.back();
             }
         }