Always add messageID to seenMessages
Always add messageID to seenMessages

This allows us to handle deduping messages keyed off outboxID and
messageID, which can happen if we receive a message from incomingMessage
and then load it via loadMoreMessages.

Thanks to Chris Nojima for suggesting this change.

keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client