fixing incorrect index reuse in AlterTableAddConstraint.java ALTER_TABLE_ADD_CONSTRAINT_REFERENTIAL and ALTER_TABLE_ADD_CONSTRAINT_UNIQUE

problem in ALTER_TABLE_ADD_CONSTRAINT_REFERENTIAL was caused by commit 08b98324 by Thomas Mueller <mueller@adobe.com>

he intended to reuse existing index... but it prevents that index from being dropped which can cause problems later
Correct solution would be to rewrite how automatically created indexes (referential/constraint) are managed to create new automatic index when dropping the reused existing index. Until this is solved it's IMHO better to not reuse existing index as it causes database to be effectively corrupted.

Also removing test code Thomas added to TestCases.java

problem in ALTER_TABLE_ADD_CONSTRAINT_UNIQUE was caused by canUseUniqueIndex allowing to use index with more columns than requested as unique index which caused unique constraint to fail as ConstraintUnique checkRow comment says that 'unique index check is enough' and the index is not unique for given constraint if it has more columns

Fix problem with websphere liberty connection pool resetting client info

Cleanup CreateCluster tool

If the database exists, it gets deleted. If the database doesn't exist, an exception gets thrown, immediately caught, and ignored. In both cases exists will be false and we continue. Later the database
gets deleted unconditionally, which means we can delete this entire block.

that I see on travis CI

Oracle doesn't support enum types

I'm not sure why there is a reference to Oracle in the discussion of https://github.com/h2database/h2database/pull/451. Oracle does not support an enum type (although it can be emulated, as in any database, using a `CHECK` constraint).

* Renamed enum NULL_CONSTRAINT to NullConstraintType
* Replaced some switches with if-statement
* Check for Oracle-Mode when parsing Oracles-SQLs NOT NULL ENABLE
VALIDATE syntax. NOTE: 
* Added DbTypeEnum-class to Mode to have a String-free dbType check
featuring the method Database.isDbTypeOneOf(DbTypeEnum... dbTypes)

Oracles NOT NULL syntax can have optional ENABLE VALIDATE added e.g  
  create table T (C int NOT NULL ENABLE VALIDATE)
Some SQL-generators produce such SQL and H2 can now parse it with this
change.

Issue#647 Removed and ignored generated file.

Issue#628 NPE in CTE