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This branch updates Tower to Tokio 0.3. Unlike #474, this branch uses Tokio 0.3's synchronization primitives, rather than continuing to depend on Tokio 0.2. I think that we ought to try to use Tokio 0.3's channels whenever feasible, because the 0.2 channels have pathological memory usage patterns in some cases (see tokio-rs/tokio#2637). @LucioFranco let me know what you think of the approach used here and we can compare notes! For the most part, this was a pretty mechanical change: updating versions in Cargo.toml, tracking feature flag changes, renaming `tokio::time::delay` to `sleep`, and so on. Tokio's channel receivers also lost their `poll_recv` methods, but we can easily replicate that by enabling the `"stream"` feature and using `poll_next` instead. The one actually significant change is that `tokio::sync::mpsc::Sender` lost its `poll_ready` method, which impacts the way `tower::buffer` is implemeted. When the buffer's channel is full, we want to exert backpressure in `poll_ready`, so that callers such as load balancers could choose to call another service rather than waiting for buffer capacity. Previously, we did this by calling `poll_ready` on the underlying channel sender. Unfortunately, this can't be done easily using Tokio 0.3's bounded MPSC channel, because it no longer exposes a polling-based interface, only an `async fn ready`, which borrows the sender. Therefore, we implement our own bounded MPSC on top of the unbounded channel, using a semaphore to limit how many items are in the channel. I factored out the code for polling a semaphore acquire future from `limit::concurrency` into its own module, and reused it in `Buffer`. Additionally, the buffer tests needed to be updated, because they currently don't actually poll the buffer service before calling it. This violates the `Service` contract, and the new code actually fails as a result. Closes #473 Closes #474 Co-authored-by: Lucio Franco <luciofranco14@gmail.com> Signed-off-by: Eliza Weisman <eliza@buoyant.io>