Websocket shutdown

[vividviolet]

Fix #70

Tophat

Run make bootstrap; make build to set up

Client close connection

1. Run ./shopify-extensions serve < testdata/shopifile.yml
2. In a web browser, open a console and paste the following code

var ws = new WebSocket("ws://localhost:8000/extensions/");
ws.onClose = console.log;
ws.close(1000, "done");

3. Verify that the console logs out a close event with the code 1000, the reason set to "done" and wasClean = true
   

Server shut down

1. Run ./shopify-extensions serve < testdata/shopifile.yml
2. In a web browser, open a console and paste the following code

var ws = new WebSocket("ws://localhost:8000/extensions/");
ws.onClose = console.log;

3. In the terminal running serve, kill the process. Note that there's a panic: send on closed channel thrown but this is not related to the websocket shutting down. We are currently not handling interrupts in the code to monitor so it tries to send a build error notification. This can be cleaned up in a later PR.
4. Verify that the console logs out a close event with the code 1000, the reason set to "server shut down" and wasClean = true
   

[t6d]

Not for this PR, but I think we should explore if client could wrap the connection and manage access to it. I'm thinking of

func newClient(conn *websocket.Conn, MessageHandler) *client {
  client := client{...}
  go client.receive()
  return &client
}
func (c *client) send(update StatusUpdate) {}
func (c *receive)
func (c *client) close()
type MessageHandler func(msg interface{})

[vividviolet]

I made a follow up issue #90

[t6d]

In future, we should pass a ctx with a deadline here as well. The shutdown should then be done as follows:

1. Attempt graceful shutdown and wait until all connections have been closed
2. Wait no longer than the deadline on the context permits

[vividviolet]

What would be set for the context deadline?