Skip already seen messages when replaying

[hloeung]

GetPostsSince will return older messages with reaction changes since LastViewedAt (query looks to be using the posts' update_at. This may be confusing as the user may think it's a duplicate, or a post out of order. Plus, we currently don't show reaction changes when relaying messages/logs so let's skip these. e.g.

$ curl -i -H 'Authorization: Bearer some-token' 'https://mychat.internal/api/v4/channels/channel_id/posts?since=1611354473840'; echo
HTTP/2 200
server: openresty/1.15.8.2
date: Sat, 23 Jan 2021 21:21:37 GMT
content-type: application/json
content-length: 1252
vary: Accept-Encoding
strict-transport-security: max-age=15724800; includeSubDomains
expires: 0
vary: Accept-Encoding
x-request-id: hm9ny96yh3...dga
x-version-id: 5.29.0.5.29.0.88ed32c9aea1c37a5f3b7d9af1b55e24.true

{"order":["n5wcpsj8p3...7fh"],"posts":{"n5wcpsj8p3...7fh":{"id":"n5wcpsj8p3...7fh","create_at":1611354253439,"update_at":1611357403391,"edit_at":0,"delete_at":0,"is_pinned":false,"user_id":"6omasxec5j...jgy","channel_id":"mpyu47h5zj...mey","root_id":"yq1or8hb4i...8hh","parent_id":"yq1or8hb4i...8hh","original_id":"","message":"<redacted>","type":"","props":{},"hashtags":"","pending_post_id":"","has_reactions":true,"reply_count":0,"metadata":{"reactions":[{"user_id":"cx1zy66pxt...pbh","post_id":"n5wcpsj8p3...7fh","emoji_name":"+1","create_at":1611357403385}]}},},"next_post_id":"","prev_post_id":""}