Constraintfixes

Address ENG-7242, ENG-7304, ENG-7305:
ENG-7242 - Fix issues caused by ALTER TABLE dropping VoltDB custom constraints in HSQL
ENG-7304 - Inability of HSQL to parse ALTER TABLE ADD CONSTRAINT with expression constraint
ENG-7305 - Allow ALTER TABLE to make column changes to tables with expression constraints/indexes if they're empty.  
So, I added black-box tests for the limit partition rows value (not the delete stmt), the assumeunique, and the constraint expressions, but you're right, it would be nice to turn this into a compile-time issue rather than a potentially-mysterious-failure-at-customer-site issue.