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.  
The sad truth is that I stole this code from elsewhere, and this part of it is actually totally non-functional here, because StatementSchema is just going to throw away all our work and call back into here leading to processAlterTableAddUniqueConstraint, which is where I stole this from.  I only added it because *every* other compile/process pair for ALTER does the same thing <hangs head in shame/>