Instead, all instance ids are now usize (like message ids). This allows
me to enforce the fact that no two instances of a Bot must have the same
id by generating the ids in the Bot.
Reusing the same id for multiple instances that send their events to the
same place can lead to race conditions depending on how events are
handled. For example, the old instance might still be shutting down
while the new instance is already connected to a room, leading to an
InstanceEvent::Stopped from the old instance that seemingly applies to
the new instance.
