Consider yielding early if we have filled buffers to some degree

[thomaseizinger]

Today, we are doing as much work as possible within poll. We may want to consider yielding early but requesting an immediate wakeup again to allow other components to read from those buffers if they are on the same task.

cc @dignifiedquire (The code you see here is not the latest stable release used in rust-libp2p as of this writing. See libp2p/rust-libp2p#3013.)