"exit" notification doesn't seem to be correctly handled

[VladimirMakaev]

When server receives "exit" notification it should exit from Server::new(stdin, stdout, socket).serve(service).await call however it doesn't seem to be happening in the testing I'm doing

To repro you can run a server from example and send "initialize", "initialized", "shutdown", "exit" to it. What happens internally is that server keeps receiving messages and serve never exits. The server does enter "Exited" state and I can see in trace message below if I send a follow up request to the server:

 ERROR tower_lsp::transport: language server has exited

It's also impossible to know that "exit" was received from the API surface

[tekumara]

Duplicate of #399