Test case
Renemae METADATA_NOT_NULL to METADATA_CONSTRAINT
Rename test for clarity
Rename another
Check all constraints during ALTER processing
Let handler decide if onUpdatePost is appropriate
Look at ForeignKeys in TableChangeValidator
Handle remaining cases. MT test.
Move ConstraintChecker (NOT NULL) into ConstraintHandler
Move OnlineHelper construction to Store and check constraints at appropriate time.
Add INDEX_CONSTRAINT change level and ensure group index path doesn't attempt to check them
Correct invalid test row
merge nathanlws: Fix ALTER ADD FOREIGN KEY when rows already exist

Merge pull request #1271 from nathanlws/sql-layer/fix-add-fk-existing-rows
https://github.com/FoundationDB/sql-layer/pull/1271