refactor: wallet: Factor out WriteVersion() from PopulateWalletFromDB()

Writing the wallet's `CLIENT_VERSION` (which indicates the last version
to have touched a wallet) needs to be done on both wallet creation and
wallet loading.

The next commit removes the `PopulateWalletFromDatabase()` call from
wallet creation, but this behavior needs to be preserved, so this commit
factors setting `CLIENT_VERSION` out of `PopulateWalletFromDatabase()`
so that wallet creation can use it in the next commit.

src/wallet/walletdb.cpp

@@ -1175,7 +1175,7 @@ DBErrors WalletBatch::LoadWallet(CWallet* pwallet)
         return result;
 
     if (!has_last_client || last_client != CLIENT_VERSION) // Update
-        m_batch->Write(DBKeys::VERSION, CLIENT_VERSION);
+        this->WriteVersion(CLIENT_VERSION);
 
     if (any_unordered)
         result = pwallet->ReorderTransactions();

src/wallet/walletdb.h

@@ -265,6 +265,9 @@ public:
 
     DBErrors LoadWallet(CWallet* pwallet);
 
+    //! Write the given client_version.
+    bool WriteVersion(int client_version) { return m_batch->Write(DBKeys::VERSION, CLIENT_VERSION); }
+
     //! Delete records of the given types
     bool EraseRecords(const std::unordered_set<std::string>& types);