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`
I can't test now, but isn't your second example missing a call to schedule(work)? and it doesn't cleanup work on completion

Notice to that the behavior of the two examples is unchanged if you remove the calls to schedule in your example

Isn't a task also a coroutine and a generalization of a function with multiple entry points? I can't really look this stuff up on my phone.