Split the notifications into DidChange and NeedsRefresh

Previously the same notification was sent for a realm having changes
available for the user to manually refresh, and for the realm being
refreshed and updated with changes. Split these into two different
notifications and better document what exactly they mean.
Rename RLMRealmNeedsRefreshNotification -> RLMRealmRefreshRequiredNotification
Improve the tests
Merge pull request #829 from realm/tg-notifications

Split the notifications into DidChange and RefreshNeeded