ENG-7242:  Partial (doesn't address constraints with expressions or
delete statements on limit partition rows).
Fix various broken issues with VoltDB custom constraints and alter
tables:
1) Make sure we copy constraint fields when they're duplicate()-ed
2) Make sure that column checks for Constraint.LIMIT type pass
ENG-7304: Teach HSQL to parse ALTER TABLE statements with constraints
on expressions.
ENG-7305: Allow dynamic ALTER TABLES that affect expression-based
constraints and indexes, but currently only on empty tables.

ENG-7242: Complete the constraint preservation testing for expressions
ENG-7242: cleanup.  I switched up which extension comment went with
which code.
Merge pull request #1850 from VoltDB/constraintfixes

Constraintfixes, ENG-7242, ENG-7304, ENG-7305, r=jhugg,cwolff