storage_manager: Update snapshot timestamp map only if a snapshot is created

The storage manager was updating the index timestamp map even when a snapshot
creation is skipped.

Change-Id: I50e490ca49e93a9b0923f6541aa79b77bd174ba0
Reviewed-on: http://review.couchbase.org/43579
Reviewed-by: Deepkaran Salooja <deepkaran.couchbase@gmail.com>
Reviewed-by: Sarath Lakshman <sarath@couchbase.com>
Tested-by: Sarath Lakshman <sarath@couchbase.com>