A first pass at updating Tokio to use `std::future`.
Implementations of `Future` from the futures crate are updated to implement
`Future` from std. Implementations of `Stream` are moved to a feature flag.
This commits disables a number of crates that have not yet been updated.
A single-producer, multi-consumer channel that only retains the _last_ sent
value. Values are broadcasted out.
This channel is useful for watching for changes to a value from multiple
points in the code base (for example, changes to a configuration value).
Taiki Endo