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.