lnrpc: use request context in WebSocket proxy
lnrpc: use request context in WebSocket proxy

The request context was not properly used to pass it along to the gRPC
endpoint which caused streaming calls to still be active on the gRPC
side even if the WS side already hung up.
We also issue an explicit close on the forwarding writer to signal when
the WS side was closed.

aakselrod/lnd,aakselrod/lnd