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/interfaces.cpp

@@ -82,7 +82,10 @@ WalletTx MakeWalletTx(CWallet& wallet, const CWalletTx& wtx)
 WalletTxStatus MakeWalletTxStatus(const CWallet& wallet, const CWalletTx& wtx)
 {
     WalletTxStatus result;
-    result.block_height = wtx.m_confirm.block_height > 0 ? wtx.m_confirm.block_height : std::numeric_limits<int>::max();
+    result.block_height =
+        wtx.state<TxStateConfirmed>() ? wtx.state<TxStateConfirmed>()->confirmed_block_height :
+        wtx.state<TxStateConflicted>() ? wtx.state<TxStateConflicted>()->conflicting_block_height :
+        std::numeric_limits<int>::max();
     result.blocks_to_maturity = wallet.GetTxBlocksToMaturity(wtx);
     result.depth_in_main_chain = wallet.GetTxDepthInMainChain(wtx);
     result.time_received = wtx.nTimeReceived;