Make TableSnapshotScanner Audience Private

From HBASE-10462 API cleanup. TableSnapshotScanner should be Audience Private. It should not be public. See HBASE-10462 for enis rationale. If private, then there needs to be a way to construct it. It is awkward since we normally construct 'Table' by getting one from a Connection. Enis suggests:

Maybe we can do a SnapshotConnection which returns a ConnectionTable, which returns the scanner. Since connection is the factory to everything, I think this approach is the most clean.

Sounds good.