"insert into table select from another table" support

added insert into table support.

This feature is intended to support bulk inserts into cstore table from regular tables. Each insert statement cause creation of a new stripe. Therefore insert command is limited to "insert into <cstore_table> from select * from <another_table>' format. eg. single row inserts like insert into <cstore_table> values (....) is not supported.

Also introduced a regression test for this feature.
1. Please put the full-stops.
2. Please start sentences with upper-case.
3. We move to next line around the 80th column.
4. We should also say that other forms of insert is not supported.

```C
		/*
		 * Only insert operation with select subquery is supported. Update,
		 * delete, and other forms of insert operation are not supported.
		 */
```

(BTW, if you want to see tab-size equal to 4 in github, you can add "?ts=4" to the url)