XBlock Peak Confusion notes

This is a laundry list of things that are maximally confusing as we approach the point where XBlocks run in edx-platform, but most of the code is still written using XModule concepts.

Old-to-new conversions:

• XModules had .location and class Location. These are implementation details of the CMS/LMS world, and pure XBlock code can't assume ids are implemented this way. We need to move away from .location, and toward usage_ids.

Confusing things:

• ParentTracker: this fundamentally is about multiple parents for blocks, but with the usage/definition split, we no longer need to have multiple parents.
• Lots of places that need an id for a block use .location.url(), which will be going away. What should replace it?