Replace async-channel (#708)

* wip wip wip

Use tokio::sync::Notify to signal to the server when a subscriber has gone away without calling unsubscribe

* Cleanup

* Fmt

* More cleanup more TODOs

* fmt

* Address a few todos

* Update core/src/server/rpc_module.rs

Co-authored-by: Niklas Adolfsson <[email protected]>

* Update ws-server/src/server.rs

Co-authored-by: Niklas Adolfsson <[email protected]>

* Fix bad merge

* Test

* fmt

* fix test

* Finish test

* Cleanup
Add a second subscription to serverless test

* Update tests/tests/integration_tests.rs

Co-authored-by: Niklas Adolfsson <[email protected]>

* simplify test

* Review feedback: avoid allocations

* cleanup

* Remove async-channel

* remove async-channel deps

Co-authored-by: Niklas Adolfsson <[email protected]>