Use Style#setState for all style-mutating Map methods

[anandthakker]

Change each of the following Map methods to delegate to Style#setState:

• addLayer
• removeLayer
• moveLayer (this will either have to wait for Make use of moveLayer in Style#setState #3649 or cause a temporary perf regression until 3649 is completed.)
• addSource
• removeSource
• setFilter
• setLayerZoomRange
• setLight

Additionally:

• Make corresponding Style methods _private.
• Remove Map#getLayer(), so that we can guarantee that all style mutations flow through setState. (Alternatively, Map#getLayer() could be changed to return a serialized version of StyleLayer instead of the class instance.)

This should bring Style's internally-public API down to { setState: (partialState: obj) => void, update: (options: obj) => void }, where setState queues a state change and update applies all queued state changes.

[anandthakker]

This is a followup to #3621

[anandthakker]

Closing this in favor of #4875