This diff makes it so for keyserver-hosted messages, we only bump last_message_for_unread_check for messageNotifyTypes.SET_UNREAD and messageNotifyTypes.NOTIF_AND_SET_UNREAD.
It has the immediate effect of preventing last_message_for_unread_check from being bumped for messageNotifyTypes.RESCIND, which is used for un-reacts to an author's message.
In later diffs we will introduce messageNotifyTypes.NONE to several message specs, which will achieve ENG-9557.
Depends on D14336