Remove async channels for pushing buffers to be sent

[Jake-Shadle]

This makes it so that enqueuing buffers to be sent is now done with PendingSends for both io-uring and tokio implementations. This means io-uring no longer needs its own tokio runtime just to drive the channel futures to transition back to sync code. Instead of a single downstream channel that can be picked up by any receiver, the sessionpool now just has a vector of pending sends that it round robins between to spread out downstream sends.

Since I wanted to remove the tokio runtime from the io-uring thread, this also meant I needed to figure out a way to get rid of the shutdown receiver that was also async, so since all loops have a PendingSends now, I just made it so that you can signal the thread that is the receiver for the pending sends to shutdown, consolidating things a bit.

[quilkin-bot]

Build Succeeded 🥳

Build Id: 93edbbdc-4a8d-450f-8e87-5ded2decc0c0

The following development images have been built, and will exist for the next 30 days:

• us-docker.pkg.dev/quilkin/ci/quilkin:0.10.0-dev-7c78a15

To build this version:

git fetch [email protected]:googleforgames/quilkin.git pull/1043/head:pr_1043 && git checkout pr_1043
cargo build