wallet: implement BerkeleyROBatch

Implement ReadKey and HasKey of BerkeleyROBatch, and Next of BerkeleyROCursor.
Also adds the containers for records to BerkeleyRODatabase so that
BerkeleyROBatch will be able to access the records.

src/wallet/migrate.cpp

@@ -21,17 +21,42 @@ bool BerkeleyRODatabase::Backup(const std::string& dest) const
 
 bool BerkeleyROBatch::ReadKey(DataStream&& key, DataStream& value)
 {
-    return false;
+    SerializeData key_data{key.begin(), key.end()};
+    const auto it{m_database.m_records.find(key_data)};
+    if (it == m_database.m_records.end()) {
+        return false;
+    }
+    auto val = it->second;
+    value.clear();
+    value.write(Span(val));
+    return true;
 }
 
 bool BerkeleyROBatch::HasKey(DataStream&& key)
 {
-    return false;
+    SerializeData key_data{key.begin(), key.end()};
+    return m_database.m_records.count(key_data) > 0;
+}
+
+BerkeleyROCursor::BerkeleyROCursor(const BerkeleyRODatabase& database, Span<const std::byte> prefix)
+    : m_database(database)
+{
+    std::tie(m_cursor, m_cursor_end) = m_database.m_records.equal_range(BytePrefix{prefix});
 }
 
 DatabaseCursor::Status BerkeleyROCursor::Next(DataStream& ssKey, DataStream& ssValue)
 {
-    return DatabaseCursor::Status::FAIL;
+    if (m_cursor == m_cursor_end) {
+        return DatabaseCursor::Status::DONE;
+    }
+    ssKey.write(Span(m_cursor->first));
+    ssValue.write(Span(m_cursor->second));
+    m_cursor++;
+    return DatabaseCursor::Status::MORE;
 }
 
+std::unique_ptr<DatabaseCursor> BerkeleyROBatch::GetNewPrefixCursor(Span<const std::byte> prefix)
+{
+    return std::make_unique<BerkeleyROCursor>(m_database, prefix);
+}
 } // namespace wallet