Core Data Concurrency update (#151)

* Refactor PersistentRepository to perform all Core Data calls in performAndWaitThrowing

- Add NSManagedObjectContext extension to handle to capture any thrown error and return the result
- Update public Repository methods to use backgroundContext.performAndWaitThrowing
- Move fetch/delete logic into private _locked helper
- Remove direct off-queue fetch/save calls to fix concurrency rule violations
- Add doc comments

* Add Test coverage

- Add NSManagedObjectContextExtensionTests
- Expand BacktraceDatabaseTests to add concurrent read/write operations, maxRecordCount and retry count Tests

Author: melekr