wallet: fully migrate address book entries for watchonly/solvable wallets

Currently `migratewallet` migrates the address book (i.e. labels and
purposes) for watchonly and solvable wallets only in RAM, but doesn't
persist them on disk. Fix this by adding another loop for both of the
special wallet types after which writes the corresponding NAME and
PURPOSE entries to the database in a single batch.

Author: theStack

src/wallet/wallet.cpp

@@ -4002,6 +4002,23 @@ bool CWallet::ApplyMigrationData(MigrationData& data, bilingual_str& error)
             }
         }
     }
+
+    // Persist added address book entries (labels, purpose) for watchonly and solvable wallets
+    auto persist_address_book = [](const CWallet& wallet) {
+        LOCK(wallet.cs_wallet);
+        WalletBatch batch{wallet.GetDatabase()};
+        for (const auto& [destination, addr_book_data] : wallet.m_address_book) {
+            auto address{EncodeDestination(destination)};
+            auto purpose{addr_book_data.purpose};
+            auto label{addr_book_data.GetLabel()};
+            // don't bother writing default values (unknown purpose, empty label)
+            if (purpose != "unknown") batch.WritePurpose(address, purpose);
+            if (!label.empty()) batch.WriteName(address, label);
+        }
+    };
+    if (data.watchonly_wallet) persist_address_book(*data.watchonly_wallet);
+    if (data.solvable_wallet) persist_address_book(*data.solvable_wallet);
+
     // Remove the things to delete
     if (dests_to_delete.size() > 0) {
         for (const auto& dest : dests_to_delete) {