DERBY-6566: Simplify handling of untyped nulls in CASE and NULLIF expressions

Make the parser represent untyped nulls the same way in CASE
expressions as in NULLIF expressions, so that ConditionalNode can
handle the two kinds of expressions uniformly.

Make the parser create a single ConditionalNode for each CASE
expression, instead of creating a tree of ConditionalNodes for a
single CASE expression.

git-svn-id: https://svn.apache.org/repos/asf/db/derby/code/trunk@1593898 13f79535-47bb-0310-9956-ffa450edef68
DERBY-532 Support deferrable constraints

Patch derby-532-metadata-queries: updates the metadata queries to give
correct results in the DEFERRABILITY column returned by the calls:

- DataBaseMetaData#getImportedKeys
- DataBaseMetaData#getExportedKeys
- DataBaseMetaData#getCrossReference.

Test have been added.

git-svn-id: https://svn.apache.org/repos/asf/db/derby/code/trunk@1593949 13f79535-47bb-0310-9956-ffa450edef68