Mappers Return EntityIterator

## Mappers Return EntityIterator

### Acceptance Criteria
1. `findAllBy` methods in `AbstractMapper` are refactored to return either an `EntityIterator`.
1. `EntityIterator` implements `Zend\Stdlib\ArraySerializableInterface`
1. Calling `EntityIterator::getArrayCopy` returns an array of arrays (as opposed to an array of entities) replacing  `AbstractRestController::nestedArrayFromEntities`.
1. Calling `EntityIterator::exchangeArray` throws a LogicException to indicate that it’s not implemented and not to use it.

### Tasks
- None

### Additional Notes
- This is a breaking change.