This fix is long coming. For a long time due to the addition of a
global user cache, the on_member_update event would only have the
updated user in the very first dispatch due to a quirk in the reference
only being updated once.
In order to fix this issue two things had to change:
1. There had to be a new event, `on_user_update` to complement
the equivalent member event.
2. Unnecessary copies of User had to be removed to compensate for the
performance hit from the diffing.
While doing these two fixes I also re-evaluated some more unnecessary
copies done during the PRESENCE_UPDATE to add member case while
fetch_offline_members=False is set or due to chunking issues. The
number of copies was brought down from 2 to 1, discounting the original
Member creation. Unsure on the benefits of this one, however.
N.B: this doesn't change the pre-existing behaviour of on_member_update
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