Move auto-reload functionality from app to Meteor Tool

[klaussner]

Problem

The functionality that reloads the page in the browser after rebuilds is implemented in the hot-code-push, autoupdate, and reload packages and uses a DDP WebSocket. Since these packages are always included in the client and server bundles, they can be used during development and in production. However, some apps only need them during development, for example if they use Apollo instead of Meteor pub/sub or if they don't use Cordova. If DDP and auto-reloading aren't used in production, this results in up to 90 kB of unused code in the client bundle and an unused DDP WebSocket connection for each active client.

Solution

We can implement development-only auto-reloading directly in the Meteor Tool by creating a WebSocket server for this functionality in Meteor's development proxy (similar to what other tools like webpack-dev-server are doing). I've started to implement a simple version of this concept as part of meteor/meteor#10505 (see the "Technical Details" and "Future Work" sections for details). This solution can be backwards-compatible and opt in: If autoupdate is included in an app, the Meteor Tool would continue to use the current system; otherwise, it would use the development proxy for auto-reloading.

Advantages

• Smaller bundles for apps that don't use DDP
• No unused WebSocket connections for apps that don't use DDP
• Enables Meteor Tool UX improvements (e.g., better error page: [wip] Improve error page meteor#10505)
• Better GraphQL subscriptions compatibility (would fix Doing this in Meteor makes the default websocket connection of meteor fail apollographql/subscriptions-transport-ws#366)
• Backwards-compatible

Disadvantages

• There are already three packages related to auto-reloading (hot-code-push, autoupdate, and reload). Implementing this FR would add another package and increase the complexity of the Meteor Tool.

[mitar]

Also interesting is that this might decrease latency for hot-reload. If client is just in the process of sending a large chunk of data, hot-reload could happen in parallel instead over a parallel connection.