rename reverse -> oldest_first, which is more obvious what it does.
Then, honor it entirely - if you specify no `after` endpoint, we default
to the beginning of message history, similar to how `before` defaults to
the end of message history.
This is a breaking change, and will change the behavior of any iterator
that previously would have been returning messages in a weird order for
limits over 100
`for msg in history(reversed=True, limit=300)` would return the newest
300 messages, in a messed up order (100..0, 200..100, 300..200).
`for msg in history(oldest_first=True, limit=300)` will now return the
oldest 300 messages in order. And so on.
`for msg in history(after=msg)` is unchanged, this previously would
return the oldest 100 messages after `msg`, oldest->newest order, and
still will.
