Tracking issue for `wait_timeout_with` stabilization

The [`wait_timeout_with` method](http://static.rust-lang.org/doc/master/std/sync/struct.Condvar.html#method.wait_timeout_with) allows you to wait on a `CondVar` with a particular timeout, but also to cancel waiting based on a closure you pass in.

It's not clear if this API is broadly useful enough to go in `std`, or whether this is its ideal form.

cc @sfackler 
