vbucket-seqno stat return keys are inconsistent with other stats

Some ep-engine stats are inconsistent. As we introduce new ones for UPR that haven't been in a release yet, I think it's still time to fix it. Here's an example:
`vbucket-details 1`:
vb_1:num_items: 10

`vbucket-seqno 1`:
vb_1_high_seqno: 107

I think `vbucket-seqno` should also have a colon after the vbucket id (instead of an underscore). If that really changes, also the spec for it needs to be updates [1].

[1] https://github.com/couchbaselabs/cbupr/blob/master/commands/stats-vbucket-seqno.md