Support 'insert' updates for cached tables (Shark-93)

For "insert into" commands, a UnionRDD combines the "select" query output with the table RDD being updated. For "insert overwrite", the output RDD replaces the old one in CacheManager.

Included tests:
- "insert into" on a single cached table
- "insert overwrite" on a single cached table
- "insert" on a cached table in a command w/ multiple inserts throws an error