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

[anandthakker]

Followup to #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.