DERBY-532: Support deferrable constraints

Patch derby-532-testAlterConstraintInvalidation.

Adds a fixture to test that prepared statement is invalidated when a
table its depends on undergoes an ALTER TABLE ALTER CONSTRAINT
statement. As it turns out, this is already handled by the common
machinery for ALTER TABLE.

git-svn-id: https://svn.apache.org/repos/asf/db/derby/code/trunk@1519045 13f79535-47bb-0310-9956-ffa450edef68