Added functionality to delete files automatically on DROP FOREIGN TABLE

This change adds functionality to allow cstore_fdw to delete files automatically on DROP FOREIGN TABLE command. It also fixes issues "cstore_fdw should error out on create if it cannot access the file" and "cstore_fdw should return empty table if no data has been loaded".
Should we explain that the goal of CStoreBeginWrite() and CStoreEndWrite() is to create valid empty table data and footer files? We can also group CStoreBeginWrite() and CStoreEndWrite() together. If we do this, we can remove the "/* end write session ... */" comment.