Send diffStyles output to Worker#updateLayers, rather than complete style layers info

Followup to https://github.com/mapbox/mapbox-gl-js/pull/3621

By adding code analogous to Style#setState on the Worker, we could reduce (sometimes significantly, I think) the amount of data that needs to be sent to workers for an updateLayers operation.

In fact, a nice way to do this would be:

• Pare Style's responsibilities down exclusively to state management (maybe renaming it to StyleState or some such). This means extracting stuff like SourceCache management, Worker updates, etc. to a higher level parent module, instead emitting state change events that the parent object could use to do the SourceCache/Worker/etc. updating stuff.
• Replace StyleLayerIndex with StyleState (adding an option allowing the worker to disable validation), and pull out the part of setState that actually applies the diff operations into a separate method.
• In StyleState, emit the diffStyles operations that were applied as part of the payload for the state change events, so that updateWorkers can just send that instead of the serialized style layers.

Start by reading pull request #3621 and tracing Style#setState, StyleLayerIndex, updateWorkers, and updateLayers. Compare the current worker payload with the proposed diffStyles operations; done means workers receive the diff instead of complete serialized style-layer information while preserving layer updates.