truncating a partitioned table with cstore_fdw partitions fail

That is probably due to us handling truncate command via process utility.

Note : Postgres does support truncating foreign tables. So we hook into process utility and modify truncate command to remove any cstore_fdw tables in it. Then we do our internal truncation.

Repro steps

```sql
create table pp(a int, b int) partition by range(a);
create table p1 partition of pp for values from (1) to (10);
create foreign table c1 partition of pp for values from (10) to (20) server cstore_server;
insert into pp values (1,1);
select * from pp;
truncate pp; -- will fail with ERROR: "c1" is not a table
-- see truncate on regular table partition is not performed either
select * from pp;
```