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`
Yes we should have something like that. However I'm not sure how it should behave. Well-behaved tasks would catch the exception, clean up, and exit, but nothing stops you from ignoring the exception. On the other hand, if we have a special uncatchable exception then you might not be able to clean up.

Also, not sure about this, but maybe we should use `InterruptException`. If it's close enough to the right thing, it might be easier than adding a new type that might need special handling.