Subsequent writes become visible to cursor when reading columnar tables

_Originally posted by @onurctirtir in https://github.com/citusdata/citus/issues/5154#issuecomment-909086250_

Below scenario is also reproduceable on older versions (at least on v10.1.2).
The root cause is that we build the `readState` when reading the first row, so we don't use the snaphot provided when running `DECLARE` command.

Even if we've called `init_columnar_read_state` in `begin_read`, we would still encounter with this issue when doing index-scan. This is because, `index_begin_read` doesn't provide a snapshot. For this reason, we have to use the snapshot passed to `index_fetch_tuple` there, which first calls `init_columnar_read_state` and that flushes the pending writes of the current xact first ((5, 6), (7, 8)).

Unfortunately, #5154 wouldn't fix that issue.

```sql
CREATE TABLE columnar_table(q1 int8, q2 int8) USING columnar;
INSERT INTO columnar_table VALUES (1, 2), (3, 4);

BEGIN;
DECLARE columnar_cursor CURSOR WITH HOLD FOR
SELECT * FROM columnar_table ORDER BY 1,2;

INSERT INTO columnar_table VALUES (5, 6), (7, 8);

-- bad, shows (5, 6) and (7, 8) as well :/
-- heapAM would only show (1, 2) & (3, 4)
FETCH ALL FROM columnar_cursor;
┌────┬────┐
│ q1 │ q2 │
├────┼────┤
│ 1 │ 2 │
│ 3 │ 4 │
│ 5 │ 6 │
│ 7 │ 8 │
└────┴────┘
(4 rows)
```