Wait event doesn't create on init

[JamesArruda]

The Wait event doesn't create _simpy_event during init, which hurts some sanity checking for MultiEvent use cases.

[JamesArruda]

There's a deeper problem here where the way _simpy_event (or the equivalents in other events) work is they make fresh versions of themselves each time as_event() is called. In SimPy, you can yield back an existing timeout and will end at the originally intended time. SimPy puts timeouts on the _queue as soon as they are created, and .callbacks are added or removed as the timeouts are yielded, or things like Any finish.

It's bad to have this mismatch between SimPy behavior and UPSTAGE behavior, so this ticket should make all events match the same behavior.