MB-8689 Add API to get list of indexed seqs per view group

Change-Id: I5f07990fd34a7ba6ac2ceda1c0f674c4592d2ddc
Reviewed-on: http://review.couchbase.org/28057
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Filipe David Borba Manana <fdmanana@gmail.com>
Tested-by: Filipe David Borba Manana <fdmanana@gmail.com>