Add an AIS validation for verifying the datatype of a generated column
is an integer type (Tiny, small, int, big). 

Add test to verify same.
merge tjoneslo: Add an AIS validation for verifying the datatype of a generated column

Merge pull request #1270 from tjoneslo/sql-layer/generator-column-only-integers
https://github.com/FoundationDB/sql-layer/pull/1270