exec: http: close websocket connection gracefully

In this loop, we ought to close the websocket connection gracefully when
the StreamErrWrapper reaches EOF.

Previously, it's possible that that we drop the last few events or skip sending
the websocket closure. If `handler(handlerPipe)` returns and `cancel` is called,
before the loop here completes processing streaming events, the loop exits
prematurely without propagating the last few events.

Instead here, the loop continues until we hit `httpPipe` EOF (through
`decoder.Decode`), to ensure we process the events to completion.

command/agent/alloc_endpoint.go

@@ -515,13 +515,6 @@ func (s *HTTPServer) execStreamImpl(ws *websocket.Conn, args *cstructs.AllocExec
 		go forwardExecInput(encoder, ws, errCh)
 
 		for {
-			select {
-			case <-ctx.Done():
-				errCh <- nil
-				return
-			default:
-			}
-
 			var res cstructs.StreamErrWrapper
 			err := decoder.Decode(&res)
 			if isClosedError(err) {