"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. Since the previous statement is assigning ```writeState``` to ```ri_FdwState```, having both of them in this comment seems redundant.
2. In other places we have the ```EXPLAIN``` and ```Explain``` spellings in similar situations, and we didn't put single quotes around them.