Improvements to Indexer Restart

- Handle the case if there is no snapshot for a slice
- Initial stream indexes cannot be recovered for now.

Change-Id: I5e9d19196afb3c895289c99247ed2545732c79a9
Reviewed-on: http://review.couchbase.org/43578
Reviewed-by: Sarath Lakshman <sarath@couchbase.com>
Reviewed-by: Deepkaran Salooja <deepkaran.couchbase@gmail.com>
Tested-by: Deepkaran Salooja <deepkaran.couchbase@gmail.com>