rename ConstraintNameFinder.findConstraintName to findConstraintNames

since it returns a List of constraints
fix ConstraintNameFinder and ForeignKeyDeletionsHandler for DB2

in contrast to H2, DB2 does not have separate foreign key names if
multiple tables reference the same parent table

example:
TABLE01(A INT)
TABLE02(A INT, FOREIGN KEY(A) REFERENCES(TABLE01.A))
TABLE03(A INT, FOREIGN KEY(A) REFERENCES(TABLE01.A))
in DB2 both foreign key constraints can have the same name,
in H2 they have different names, at least when defined as above
only store rows which could have children in ForeignKeyDeletionsHandler

this is to avoid out of memory errors for large data sets
and vast amounts of deleted rows
add Anonymizer command line argument to skip rule validation
remove some TODO and unused methods