DERBY-6169: Reduce visibility of classes and methods under impl/sql

- Remove the unused targetColumns fields in CursorInfo and
  GenericPreparedStatement

- Replace the getter for the updateColumns field in
  GenericPreparedStatement with more specific accessor methods that
  don't expose the underlying array

- Make CursorInfo.updateColumns a list instead of an array to take
  advantage of helper methods in java.util.List

- Make ArrayUtil.readStringArray() create one array rather than two

- Move QueryTreeNode.getCursorInfo() to StatementNode, since it's only
  called on nodes of that type

git-svn-id: https://svn.apache.org/repos/asf/db/derby/code/trunk@1470478 13f79535-47bb-0310-9956-ffa450edef68