wallet: add dummy BerkeleyRODatabase and BerkeleyROBatch classes

BerkeleyRODatabase and BerkeleyROBatch will be used to access a BDB file
without the use of BDB. For now, these are dummy classes.

src/wallet/migrate.cpp

@@ -0,0 +1,37 @@
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+#include <wallet/migrate.h>
+
+namespace wallet {
+
+void BerkeleyRODatabase::Open()
+{
+}
+
+std::unique_ptr<DatabaseBatch> BerkeleyRODatabase::MakeBatch(bool flush_on_close)
+{
+    return std::make_unique<BerkeleyROBatch>(*this);
+}
+
+bool BerkeleyRODatabase::Backup(const std::string& dest) const
+{
+    return false;
+}
+
+bool BerkeleyROBatch::ReadKey(DataStream&& key, DataStream& value)
+{
+    return false;
+}
+
+bool BerkeleyROBatch::HasKey(DataStream&& key)
+{
+    return false;
+}
+
+DatabaseCursor::Status BerkeleyROCursor::Next(DataStream& ssKey, DataStream& ssValue)
+{
+    return DatabaseCursor::Status::FAIL;
+}
+
+} // namespace wallet