wallet: Avoid use of Chain::Lock in importwallet and dumpwallet

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

This change only affects behavior in the case where wallet last block processed
falls behind the chain tip, in which case it will use more accurate backup and
rescan timestamps.

src/wallet/test/wallet_tests.cpp

@@ -225,6 +225,7 @@ BOOST_FIXTURE_TEST_CASE(importwallet_rescan, TestChain100Setup)
         request.params.setArray();
         request.params.push_back(backup_file);
         AddWallet(wallet);
+        wallet->SetLastBlockProcessed(::ChainActive().Height(), ::ChainActive().Tip()->GetBlockHash());
         ::dumpwallet(request);
         RemoveWallet(wallet);
     }
@@ -233,16 +234,17 @@ BOOST_FIXTURE_TEST_CASE(importwallet_rescan, TestChain100Setup)
     // were scanned, and no prior blocks were scanned.
     {
         std::shared_ptr<CWallet> wallet = std::make_shared<CWallet>(chain.get(), WalletLocation(), WalletDatabase::CreateDummy());
+        LOCK(wallet->cs_wallet);
         wallet->SetupLegacyScriptPubKeyMan();
 
         JSONRPCRequest request;
         request.params.setArray();
         request.params.push_back(backup_file);
         AddWallet(wallet);
+        wallet->SetLastBlockProcessed(::ChainActive().Height(), ::ChainActive().Tip()->GetBlockHash());
         ::importwallet(request);
         RemoveWallet(wallet);
 
-        LOCK(wallet->cs_wallet);
         BOOST_CHECK_EQUAL(wallet->mapWallet.size(), 3U);
         BOOST_CHECK_EQUAL(m_coinbase_txns.size(), 103U);
         for (size_t i = 0; i < m_coinbase_txns.size(); ++i) {