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/transaction.cpp

@@ -15,7 +15,7 @@ bool CWalletTx::IsEquivalentTo(const CWalletTx& _tx) const
 
 bool CWalletTx::InMempool() const
 {
-    return fInMempool;
+    return state<TxStateInMempool>();
 }
 
 int64_t CWalletTx::GetTxTime() const