wallet: Introduce DatabaseCursor RAII class for managing cursor

Instead of having DatabaseBatch deal with opening and closing database
cursors, have a separate RAII class that deals with those.

For now, DatabaseBatch manages DatabaseCursor, but this will change
later.

src/wallet/sqlite.h

@@ -14,19 +14,27 @@ struct bilingual_str;
 namespace wallet {
 class SQLiteDatabase;
 
+class SQLiteCursor : public DatabaseCursor
+{
+public:
+    sqlite3_stmt* m_cursor_stmt{nullptr};
+
+    explicit SQLiteCursor() {}
+    ~SQLiteCursor() override;
+
+    bool Next(CDataStream& key, CDataStream& value, bool& complete) override;
+};
+
 /** RAII class that provides access to a WalletDatabase */
 class SQLiteBatch : public DatabaseBatch
 {
 private:
     SQLiteDatabase& m_database;
 
-    bool m_cursor_init = false;
-
     sqlite3_stmt* m_read_stmt{nullptr};
     sqlite3_stmt* m_insert_stmt{nullptr};
     sqlite3_stmt* m_overwrite_stmt{nullptr};
     sqlite3_stmt* m_delete_stmt{nullptr};
-    sqlite3_stmt* m_cursor_stmt{nullptr};
 
     void SetupSQLStatements();
 
@@ -44,9 +52,7 @@ public:
 
     void Close() override;
 
-    bool StartCursor() override;
-    bool ReadAtCursor(CDataStream& key, CDataStream& value, bool& complete) override;
-    void CloseCursor() override;
+    std::unique_ptr<DatabaseCursor> GetNewCursor() override;
     bool TxnBegin() override;
     bool TxnCommit() override;
     bool TxnAbort() override;