bot: threading.Event instead of Boolean for `_connection_registered`

[dgw]

Threading.Event could be used for _connection_registered instead of a boolean

This is a neat idea and worth looking into, largely because of the .wait() method that would make available.

Originally posted by @dgw in #2375 (comment)

I want to come back to this idea in the future, maybe for 9.0. (It's not the sort of thing I'd be comfortable changing in a minor release like 8.1.)

[dgw]

(When making #2406 I realized that this doesn't change anything. bot.connection_registered changed from a Boolean to a property in #2375. That's what might actually affect third-party code, so I moved the Breaking Change label to that PR.)