Always use stored last viewed at if present

This partially reverts commit 3a0d0b3. We can't trust that Mattermost
has updated the last viewed for channel which is why we store the
LastViewedAt internally (see PR#361 and #245).

mm-go-irckit/userbridge.go

@@ -591,13 +591,13 @@ func (u *User) addUserToChannelWorker(channels <-chan *bridge.ChannelInfo, throt
 		if since == 0 {
 			continue
 		}
-		// We used to stored last viewed at if present.
+
+		// We use the stored LastViewedAt if present as we can't
+		// always trust that the Mattermost server has updated the
+		// last viewed at for the channel (PR#361).
 		u.lastViewedAtMutex.RLock()
 		if lastViewedAt, ok := u.lastViewedAt[brchannel.ID]; ok {
-			// But only use the stored last viewed if it's later than what the server knows.
-			if lastViewedAt > since {
-				since = lastViewedAt + 1
-			}
+			since = lastViewedAt + 1
 		}
 		u.lastViewedAtMutex.RUnlock()
 		// post everything to the channel you haven't seen yet