datastore: versioned mutations

**Is your feature request related to a problem? Please describe.**
Looking through the internal code of the Datastore library, I notice that the LookupResult and Mutation both have properties with a 'version' field. The entities in the Datastore apparently have a version and it would be possible to create mutations that are rejected because the entity is outdated.

This kind of optimistic locking is precisely what I'd like to use. However, the current Datastore library does not expose this.

**Describe the solution you'd like**
Just like the Datastore currently supports loading Key's into `__key__` labeled fields (KeyLoader), the library could also support loading the version into `__version__` labeled fields (VersionLoader). Then any update/mutation could reinstantiate this `__version__` value into the Mutation.

**Describe alternatives you've considered**
Alternatives like extending the user API via options would be troublesome and cumbersome for existing users of the library to implement. The `__version__` addition would be fairly invisible for existing users. Through the documentation we can explain this feature.