wallet: Avoid use of Chain::Lock in CWallet::ScanForWalletTransactions

This is a step toward removing the Chain::Lock class and reducing cs_main
locking.

This change affects behavior in a few small ways.

- If there's no max_height specified, percentage progress is measured ending at
  wallet last processed block instead of node tip

- More consistent error reporting: Early check to see if start_block is on the
  active chain is removed, so start_block is always read and the triggers an
  error if it's unavailable

src/qt/test/wallettests.cpp

@@ -153,7 +153,7 @@ void TestGUI(interfaces::Node& node)
     {
         WalletRescanReserver reserver(wallet.get());
         reserver.reserve();
-        CWallet::ScanResult result = wallet->ScanForWalletTransactions(Params().GetConsensus().hashGenesisBlock, {} /* stop_block */, reserver, true /* fUpdate */);
+        CWallet::ScanResult result = wallet->ScanForWalletTransactions(Params().GetConsensus().hashGenesisBlock, 0 /* block height */, {} /* max height */, reserver, true /* fUpdate */);
         QCOMPARE(result.status, CWallet::ScanResult::SUCCESS);
         QCOMPARE(result.last_scanned_block, ::ChainActive().Tip()->GetBlockHash());
         QVERIFY(result.last_failed_block.IsNull());