Migrate to NSOperation-based network requests.

- reduce poll iteration count
- change request URL for testing
- fix typo

- remove the need for using NSLock to update `state`
- remove the need for using NSLock to update `currentTask`
- add KVO dependencies from `state` -> `isFinished` & `isExecuting`

- remove duplicate compile sources
- sync dynamic and static framework umbrella headers
- rename BUYClient+Checkout helpers category to avoid duplicate name