Simplify Shark table properties.

All tables are unified by default and all cached tables are stored at
`StorageLevel.MEMORY_AND_DISK`. These are the new"shark.cache" options:
- "memory": cache in memory and store on disk. These tables are
  persistent, and automatically reloaded across Shark session restarts.
- "memory_only": cache in memory only. These tables are dropped at
  the end of a Shark session
- "none": Hive table