wallet: automatic birth time update

As the user could have imported a descriptor with
a newer timestamp (by blindly setting 'timestamp=now'),
the wallet needs to update the birth time when it detects
a transaction older than the oldest descriptor timestamp.

src/wallet/wallet.cpp

@@ -1080,6 +1080,9 @@ CWalletTx* CWallet::AddToWallet(CTransactionRef tx, const TxState& state, const
         wtx.m_it_wtxOrdered = wtxOrdered.insert(std::make_pair(wtx.nOrderPos, &wtx));
         wtx.nTimeSmart = ComputeTimeSmart(wtx, rescanning_old_block);
         AddToSpends(wtx, &batch);
+
+        // Update birth time when tx time is older than it.
+        FirstKeyTimeChanged(/*spkm=*/nullptr, wtx.GetTxTime());
     }
 
     if (!fInsertedNew)
@@ -1199,6 +1202,10 @@ bool CWallet::LoadToWallet(const uint256& hash, const UpdateWalletTxFn& fill_wtx
             }
         }
     }
+
+    // Update birth time when tx time is older than it.
+    FirstKeyTimeChanged(/*spkm=*/nullptr, wtx.GetTxTime());
+
     return true;
 }
 
@@ -3087,7 +3094,7 @@ std::shared_ptr<CWallet> CWallet::Create(WalletContext& context, const std::stri
         int64_t time = spk_man->GetTimeFirstKey();
         if (!time_first_key || time < *time_first_key) time_first_key = time;
     }
-    if (time_first_key) walletInstance->m_birth_time = *time_first_key;
+    if (time_first_key) walletInstance->FirstKeyTimeChanged(/*spkm=*/nullptr, *time_first_key);
 
     if (chain && !AttachChain(walletInstance, *chain, rescan_required, error, warnings)) {
         return nullptr;