Splat: Squashed concurrency bugs

Add the optimistic concurrency tests, and fix bugs they revealed. 

:bug: fixed:
- Ats query pipeline support for nullable types
- Query string type mismatch when querying on rowkey and partitionkey
- Concurrency errors when row has been deleted

(Will squash to one commit before merge.)
Do you actually need to specify the Key again? If so this might be a bug.