Creating other FDW table generates files in $PGDATA/cstore_fdw/(database's oid)

When I created a mysql_fdw table after setting up cstore_fdw, I found data file and footer file were generated in $PGDATA/cstore_fdw/(database's oid).

```
datatank=> CREATE SERVER mysql_server
datatank-> FOREIGN DATA WRAPPER mysql_fdw
datatank-> OPTIONS (host '127.0.0.1', port '3306');
CREATE SERVER
datatank=> CREATE USER MAPPING FOR postgres
datatank-> SERVER mysql_server
datatank-> OPTIONS (username 'foo', password 'bar');
CREATE USER MAPPING
datatank=> CREATE FOREIGN TABLE warehouse(
datatank(> warehouse_id int,
datatank(> warehouse_name text)
datatank-> SERVER mysql_server
datatank-> OPTIONS (dbname 'db', table_name 'warehouse');
CREATE FOREIGN TABLE
datatank=> select relfilenode from pg_class where relname = 'warehouse' and relkind = 'f';
relfilenode
-------------
16454
(1 row)

/mnt/postgresql/9.5/main/cstore_fdw/16386# ls -ltr
total 404
-rw------- 1 postgres postgres 54 Jun 20 02:40 16422.footer
-rw------- 1 postgres postgres 397099 Jun 20 02:40 16422
-rw------- 1 postgres postgres 26 Jun 21 09:00 16454.footer
-rw------- 1 postgres postgres 0 Jun 21 09:00 16454
```