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/wallet/rpcwallet.cpp

@@ -3569,7 +3569,7 @@ UniValue rescanblockchain(const JSONRPCRequest& request)
     }
 
     CWallet::ScanResult result =
-        pwallet->ScanForWalletTransactions(start_block, stop_height, reserver, true /* fUpdate */);
+        pwallet->ScanForWalletTransactions(start_block, start_height, stop_height, reserver, true /* fUpdate */);
     switch (result.status) {
     case CWallet::ScanResult::SUCCESS:
         break;