When we receive snapshot, we only add and merge new entries.
Part of: https://gitlab.com/gitlab-org/gitter/webapp/issues/2276
We used to remove all elements that weren't included in the snapshot. That's not playing nicely with only sending afterId
snapshots instead of the whole response (50 for messages).
Only change is in 41188ce3 I'll rebase this onto master once !25 (merged) gets merged
Edited by Tomas Vik