Simplify and consolidate Shape sources

[asheemmamoowala]

In #9983 and previous tickets, there has been much discussion over the naming and convention around MGLComputedShapeSource and its related classes.

Previous conversations have centered around:

• Removing MGLAbstractShapeSource in favor of a single MGLShapeSource that allows providing the shape content in a single call, or via callbacks. #9983(comment)
• Improving the API interface for MGLComputedShapeSourceDataSource to better indicate that it will not be called on the main thread, and should be implemented with thread safety in mind.1, 2, and 3
• Simplifying MGLShapeSourceOption, and better align it with the specific options of the shape source being used.

cc @1ec5

[1ec5]

iOS SDK v4.0.0 may be our last opportunity to implement any of these options for awhile, since v5.0.0 would presumably be further down the road.

/cc @lilykaiser

[1ec5]

#10853 may be a way to pare back this work to something manageable in the v4.0.0 timeframe.

[1ec5]

#11568 removed MGLAbstractShapeSource, but it left separate MGLShapeSource and MGLComputedShapeSource classes. A future unification – or at least making one a subclass of the other – would be desirable, but any changes along these lines would require refactoring the underlying mbgl classes as well.