Batch mode for efficiently mutating Style instances

[scothis]

Mutation operations on a Style instance are designed to be atomic.
Depending on the operation, there is a significant amount of work that
must be preformed to flush and recreate state. This work is wasted when
multiple mutation methods are invokes sequentially.

Batching these mutations allows deferring this work until the batch is
complete and deduping the work.

map.batch(function(batch) {
    batch.addLayer(layer1);
    batch.addLayer(layer2);
    ...
    batch.addLayer(layerN);
});

Methods on the batch object provided to the work function mirror methods
on Map and Style, and include:

• addLayer
• removeLayer
• setPaintProperty
• setLayoutProperty
• setFilter
• addSource
• removeSource

Internally, calls to four methods are deferred:

• style._groupLayers (will only fire once)
• style._broadcastLayers (will only fire once)
• style._reloadSource (will fire once per source)
• style.fire (each call will fire)

A casual benchmark shows the time to add 80 layers dropping from ~800 ms
to ~20 ms when run as a batch.

Issue: #1341

[jfirebaugh]

This looks great. Only adjustment I suggest is to ensure that a style change event fires only once per batch.

[scothis]

@jfirebaugh the 'change' event is fired at most once per batch now

[mourner]

A bit late to say that this is a really awesome PR.