First pass at fixing SQL Server 2000 pagination

This is an attempt to restore SQL Server 2000 pagination while reusing the existing local variables.
In general, I think it's undesirable for getters to have side effects (e.g., setting the left/right sql quotes in is_db2). Here's what I would suggest:

```python
@property
def is_db2(self):
    if self._is_db2 is None:
        cur = self.connection.cursor()
        try:
            cur.execute("SELECT * FROM SYSIBM.COLUMNS FETCH FIRST 1 ROWS ONLY")
            self._is_db2 = True
        except Exception:
            self._is_db2 = False

    return self._is_db2

@property
def left_sql_quote(self):
    if self.is_db2:
        return '{'
    else:
        return '['

@property
def right_sql_quote(self):
    if self.is_db2:
        return '}'
    else:
        return ']'
```

With this we can also eliminate the private attributes `_left_sql_quote` and `_right_sql_quote`.