Support single row inserts with column values

cstore_fdw should support insert statements in the form of

```
insert into cstore_table values(value1, value2....)
insert into cstore_table(column1, column2...) values(value1, value2....)
insert into cstore_table values(value1, value2....), (othervalue1, othervalue2,...) ...
insert into cstore_table(column1, column2...) values(value1, value2....), (othervalue1, othervalue2,...) ...
```

1 - single row insert with all column values
2 - single row insert with selected column values
3 - multiple row insert with all column values
3 - multiple row insert with selected column values

Creation of a new stripe for each insert statements was the reason preventing single row inserts. Now we need to change that behavior. Solution would probably look like loading the last stripe for the insert, appending to it and saving it back. Concurrent inserts should also be considered.