feat(wsapi): Gracefully close the websocket connection

[armsnyder]

This PR removes a time.Sleep(1 * time.Second) from Session.Close().

Instead, I implemented the websocket close handshake described in this comment: gorilla/websocket#448 (comment)

The reason I would like to see this change is that the time.Sleep was causing my unit tests to run slowly.

I tested this change by running this program a few times:

package main

import (
	"github.com/bwmarrin/discordgo"
	"github.com/samber/lo"
)

func main() {
	session := lo.Must(discordgo.New(Getenv("BOT_TOKEN")))
	session.LogLevel = discordgo.LogDebug
	lo.Must0(session.Open())
	lo.Must0(session.Close())
}

[FedorLap2006]

After a bit of thinking. It might be a good idea to also move this function away from Session into util.go, with websocket prefixed name (e.g. doWebsocketCloseHandshake). Since we can reuse it in our voice websocket handling code.

[FedorLap2006]

setting read deadline

I'm not quite sure what do you refer to. The handlers in websocket by default don't extend the read deadline. And we don't override them (well, close is overriden, but is noop)