PoWeb server: WS socket is suddenly closed after sending first parcel

[gnarea]

And the close event isn't fired, so we can't handle it.

Since we'd be trying to write to a closed stream, that crashes the app with:

Error: WebSocket is not open: readyState 3 (CLOSED)
    at sendAfterClose (/home/gus/repos/awala-gateway-desktop/packages/ui/app/node_modules/daemon/node_modules/ws/lib/websocket.js:763:17)
    at WebSocket.send (/home/gus/repos/awala-gateway-desktop/packages/ui/app/node_modules/daemon/node_modules/ws/lib/websocket.js:345:7)
    at Duplex.duplex._write (/home/gus/repos/awala-gateway-desktop/packages/ui/app/node_modules/daemon/node_modules/ws/lib/stream.js:169:8)
    at writeOrBuffer (internal/streams/writable.js:358:12)
    at Duplex.Writable.write (internal/streams/writable.js:303:10)
    at Object.sink (/home/gus/repos/awala-gateway-desktop/packages/ui/app/node_modules/daemon/node_modules/stream-to-it/sink.js:71:20)

I can reproduce this reliably by running the ping command twice (awala-ping ping).

[gnarea]

This could have the same root cause as relaycorp/awala-gateway-internet#463