Add functionality to automatically determine filename.

This change allows cstore_fdw to automatically determine the file name used to store its files.

This change makes the filename parameter optional and will use it if specified. If filename is not specified, then cstore_fdw would use $PGDATA/cstore_fdw/{database-oid}/{relfilenode} as the path for storing its files
Minor. We usually note in the comments of the functions if they are unchanged or based on a function from CitusDB's source code. Just noting to add this in a later commit.