Beginning a new TestContextManager to assist better unit testing

No incremental upgrade changes yet
Readded CoreDataConcurrencyTest to the test target
Make saveContext a class method rather than an instance method
Switched back to instance save methods on ContextManager

Also removing CoreDataTestHelper and it's horrible swizzled methods.
Fixed problem with shared instance being nil unexpectedly

Disabled two unit tests because they misbehave with Notifications being fired off
Added ALIterativeMigrator and unit tests for 19 -> 21 migrations
Iteratively migrate a store to prevent missed mappings
Unit tests the iterative migration in ContextManager
Merge branch 'release/4.5' into issue/2357-incremental-core-data

Conflicts:
	WordPress/WordPress.xcodeproj/project.pbxproj
Fixed model mapping from 20 to 21 to account for updated model 21
Fixed a left-over of a reversed refactor
Calculate sorted model names instead of a hard-coded list
Another refactor of ContextManager leftover missed
Extract model path into a method for reuse
Merge pull request #2586 from wordpress-mobile/issue/2357-incremental-core-data

Implement iterative Core Data model upgrades