merge MMcM: Fix NULL UNION NULL handling

Merge pull request #1089 from MMcM/sql-layer/union-null-data-types
https://github.com/FoundationDB/sql-layer/pull/1089
Enough of the basic data structure to get from CREATE TABLE to AIS (via Protobuf).
Some ALTER support.
Emulated meta-data.
Clean up error handling.
I_S tables.
Add a YAML test for those DDL operations.
Sketch the overall flow.
Clarify multi-column conditions and add a test.
Handle RESTRICT and outline more details of other cases.
Take care of TODO.
Make error messages consistent. Add YAML tests for RESTRICT cases.