Clean up DocumentPersister's queued inserts on error

When users encounter such an exception, they should clear the UoW via the DocumentManager::clear() method; however, we need to ensure that related classes clean up their dirty state as well. If an insertion resulted in a MongoException, the DocumentPersister's $queuedInserts property might hold stale data, which could get re-used on a future batch insertion.

Closes #580
Remove unused properties in DocumentPersister