DERBY-6576 A immediate Fk constraint blows up iff its referenced PK is deferred and we modify a duplicate key column

A follow-up patch, derby-6576-repeatable-read. In the case where we do
not throw an exception because the deferred unique/pk constraint
referenced by an fk is upheld by another row, we need to make sure
that condition holds until we commit. This patch changes the check (in
ReferencedKeyRIChecker#isDuplicated) to scan using repeatable read
isolation level instead of read committed. This will set a read lock
on any (well, the first we find) duplicate row till transaction end
and thus ensure things are good till we commit.

git-svn-id: https://svn.apache.org/repos/asf/db/derby/code/trunk@1600779 13f79535-47bb-0310-9956-ffa450edef68