[ASMapNode] Greatly improve the internal logic of ASMapNode. Also fixes bug #971

[aaronschubert0]

This fixes #971 as raised by @tomizimobile. It also greatly improves the internal logic of ASMapNode. Most notably:

• You can now set _liveMap = YES on a background thread, and it will only be actioned when that node comes on screen.
• Added asserts to handle layer backing scenarios.
• Added a few additional comments to improve documentation.

Will add an example app and tests in a future diff (soon :D )
@appleguy

[appleguy]

@aaronschubert0 awesome, thanks for the ultra fast turnaround! This component has already attracted a variety of users, and in January it will be put into the new Pinterest too :).

[appleguy]

@aaronschubert0 I didn't realize until now all the options that MKMapSnapshotOptions provides - wow! Later we should consider adding some more passthroughs. I'm pretty sure we can treat that object as thread safe, so it may actually be ideal to just change the region property into the MKMapSnapshotOptions object (I had not realized this is how the snapshotter itself gets the region until converting some Pinterest code to use ASMapNode, and then noticed it would be an even more convenient conversion if it just took the same object).

[appleguy]

#988

[aaronschubert0]

@appleguy Wow!! That's super awesome. Can't wait for that update! Good start to the New Year.