Followup to discussion in #issue #1311 (and it's PR). Currently RFC doesn't explicitly forbids FullJIDs in roster, but the general consensus is that it's very, very bad idea and should be avoided altogether. To that end we should switch our RosterAPI to use BareJIDs in 9.0

While we would be changing the API we could also make some additional improvements: use collections instead of arrays and avoid returning nulls in favour of empty collections.