Remove uses of cs_main in wallet code

This commit does not change behavior.

It is easiest to review this change with:

    git log -p -n1 -U0

src/qt/test/wallettests.cpp

@@ -143,7 +143,7 @@ void TestGUI()
         wallet->AddKeyPubKey(test.coinbaseKey, test.coinbaseKey.GetPubKey());
     }
     {
-        LOCK(cs_main);
+        auto locked_chain = wallet->chain().lock();
         WalletRescanReserver reserver(wallet.get());
         reserver.reserve();
         wallet->ScanForWalletTransactions(chainActive.Genesis(), nullptr, reserver, true);