Support DynamoDBVersion attribute

[darkms]

DynamoDb.DataModel's [DynamoDBVersion] attribute is used for Optimistic locking.
Not only it can be used for Update operations, but also for Delete as well (throwing exception if record was modified/no longer exists).

Auto-updating this field after SubmitChanges (so we don't have to re-query entity from table after update) would also be very nice.

Docs:
http://docs.aws.amazon.com/amazondynamodb/latest/developerguide/VersionSupportHLAPI.html