ENG-7224: Improve support for ASSUMEUNIQUE with live DDL:
- Alter table allows assumeunique in place of unique when adding
constraints
- Replicated tables may have non-partition column assumeunique constraints, allowing them
to be converted to partitioned tables.
Merge pull request #1807 from VoltDB/eng7224-part1

ENG-7224: Improve support for ASSUMEUNIQUE with live DDL: