wallet: Avoid use of Chain::Lock in importmulti

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 may use a more accurate rescan
time.

src/wallet/test/wallet_tests.cpp

@@ -152,6 +152,7 @@ BOOST_FIXTURE_TEST_CASE(importmulti_rescan, TestChain100Setup)
     {
         std::shared_ptr<CWallet> wallet = std::make_shared<CWallet>(chain.get(), WalletLocation(), WalletDatabase::CreateDummy());
         wallet->SetupLegacyScriptPubKeyMan();
+        WITH_LOCK(wallet->cs_wallet, wallet->SetLastBlockProcessed(newTip->nHeight, newTip->GetBlockHash()));
         AddWallet(wallet);
         UniValue keys;
         keys.setArray();