Refactor Style object hierarchy

[kkaefer]

Our current Style object hierarchy is a little messy: Worker threads depend on it being immutable, but it really isn't. We don't have a clear concept for changing/swapping styles, and not a clear object ownership.

• Runtime styling API #837: programmatic style API
• Get rid of shared_ptr for Source objects #904: Get rid of shared_ptr for Source objects
• Don't parse Style object in the UI thread #883: Don't parse Style object in the UI thread
• get/setStyleJSON should not be public API #900: get/setStyleJSON should not be public API
• Every tile parser thread needs its own Style object #881: Every tile parser thread needs its own Style object

[ljbade]

My ideas:

• make style immutable, ie all the change functions return a new style object copied from the current with the changes
• have a single 'master' style object in map that gets atomically swapped (or locking swap)
• every worker job gets a copy of the 'master' style when it is created
• to update/load new styles you create the new style object with the change functions, then call some function that switches out the 'master' style

[mikemorris]

Mutable styles could be responsible for https://github.com/mapbox/node-mapbox-gl-native/issues/64

[jfirebaugh]

The dependent bugs above are now fixed. Remaining actions tracked in #667.