Actors don't properly clean up callback references

[tgecho]

mapbox-gl-js version: 0.40.1

Steps to Trigger Behavior

1. Load any map (for example: https://www.mapbox.com/mapbox-gl-js/examples/)
2. Count the number of callbacks retained by the worker actors:
   On the example page, run this in the console: document.getElementById('demo').contentWindow.map.style.dispatcher.actors.reduce((total, actor) => Object.keys(actor.callbacks).length + total, 0)

Expected Behavior

The number of callbacks should rise and fall back to 0

Actual Behavior

The number of callbacks continues to climb as you interact with the map

Background / Details

I'm investigating some an apparent memory leaks (in a much more complicated scenario with auto refreshing data), and I noticed references were being held open by this issue. Basically, the actor saves a callback, but in some cases the worker never calls back since many if its methods don't use the done param. The primary culprits seem to be setLayers and updateLayers in source/worker.js, though I noticed others.

Experimentally clearing them with map.style.dispatcher.actors.forEach(a => a.callbacks = {}) seems to somewhat bring memory usage back down, though not completely.

[jfirebaugh]

Good catch!

[tgecho]

I'm trying my hand at a fix for this and I've noticed that loadTile is another leaky event type. It has a callback, so retaining it is correct, but apparently it doesn't always get called back, especially when zooming/panning.