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.)
This is intentional. The tests don't work when using composite keys. We have to do the same thing for Northwind.