Use PG internal file (relfilenode/page) structure to store cstore data

Cstore_fdw stores its meta-data and data files on the file system in its own folder. This has becoming a problem for cstore. If we can start using internal file structure we can
- drop superuser requirement since we will be staying inside pg landscape
- support pg_basebackup
- support streaming replication via WAL logging
- survive PG version upgrade without data loss or corruption.

There is a prototype available at branch [store_data_in_internal_storage](https://github.com/citusdata/cstore_fdw/tree/store_data_in_internal_storage) . It needs some refinement.

We may need to implement a migration UDF to move data stored in file system to internal storage.