backup utility may not capture highseqnos from some vbuckets

While analyzing code for Sync Gateway, I found some issues with the ergonomics of the go API, and filed GOCBC-1385

I looked to other places in couchbase org that use this function and found the implementation in backup here https://cs.github.com/couchbase/backup/blob/5bfe9f92b6b5a5958fd0975bab3af71b5dce27cb/couchbase/dcp_client.go#L318

The long term fix would be to uptake the not yet created go API at the SDK level, but a short term fix could be to just change the iteration from 1 -> numServers + 1, since the zero value is a duplicate of another value in the server array.