poll_fd fix

@JeffBezanson This implements julep #4401 in order to fix the issues with poll_fd (#6563)

julep #4401 is implemented by separating the steps involved with a call to `wait` into their component parts:

`waitq` --> prepares the wait call
`waitresult` --> extract a result from wait (if present)
~~`waitcleanup` --> cleanup after the wait call (remove from waitq)~~
`waitkill` --> ~~additional~~ cleanup (destructors) after the wait call

if these specialized methods don't exist, it simply falls back to calling `wait` in a Task (graceful degradation means a loss in performance, but not in functionality)

client code can plug into these methods, just like they currently can extend `wait`. for example, see `Timer`

also notable: `sleep(::Real)` is now an alias for `wait()`, since `wait(::Real)` was defined to mean `wait for timeout`
But the *event* of function return is not associated with the function *object*.
`wait` is about state, not just a method definition. A function object does not have any state.

The problem is very simple: for a function object `f`, multiple calls to `wait(f)` do not wait for the same thing, despite the argument being the same. Each call to wait waits for a *different* function return "event", and the function is called multiple times. This is not how `wait` should behave.