Server streaming handler not cancelled on client disconnect

[wolves-fc]

Issue:

When a client disconnects during a server streaming RPC, the async generator is never closed. The generator continues yielding indefinitely and each yield produces a failed socket write and an asyncio warning:

WARNING:asyncio:socket.send() raised exception

Expected (or at least what I think should be happening):

the framework should monitor receive() for http.disconnect and call aclose() or something along those lines.

Reproduce:

1. Create server streaming endpoint a generator that yields indefinitely (e.g. a watch rpc)
2. Connect to it with a client
3. Kill the client
4. Observe the server logging the socket.send() raised exception indefinitely

Workaround:

ASGI middleware that queues receive()

[stefanvanburen]

hey @wolves-fc, put up #175 to try to address this. LMK if anything doesn't look right over there.

[wolves-fc]

@stefanvanburen worked great for my streaming rpc. thanks!