scripted-diff: Wallet: Rename mapAddressBook to m_address_book

Previous versions assumed absence of an entry in mapAddressBook indicated change.
This no longer holds true (due to bugs) and will shortly be made intentional.
Renaming the field helps ensure that old code using mapAddressBook directly gets checked for necessary rebasing.

-BEGIN VERIFY SCRIPT-
sed -i -e 's/mapAddressBook/m_address_book/g' $(git grep -l 'mapAddressBook' ./src)
-END VERIFY SCRIPT-

src/qt/test/addressbooktests.cpp

@@ -97,7 +97,7 @@ void TestAddAddressesToSendBook(interfaces::Node& node)
 
     auto check_addbook_size = [&wallet](int expected_size) {
         LOCK(wallet->cs_wallet);
-        QCOMPARE(static_cast<int>(wallet->mapAddressBook.size()), expected_size);
+        QCOMPARE(static_cast<int>(wallet->m_address_book.size()), expected_size);
     };
 
     // We should start with the two addresses we added earlier and nothing else.