DERBY-6125 Code clean up in client driver.

derby-6125-imports-01-a. This "horizontal" patch makes the import
policy consistent for the client driver:

- iff a class from java.sql.* can be shadowed by a Derby class
  (i.e. Clob, ResultSet); we do not import such java.sql.* classes but
  refer to them using the full package and class name. The Derby
  classes are always imported and referenced in the short form. This
  makes the code much more readable IMHO.

- I'd like to change the names of the Derby classes that can shadow
  java.sql.* classes (so we can import such classes from java.sql.*
  too), but I need to investigate whether that can have any
  compatibility issues first. Does anyone know?

- removes unused public static variable ClientXid#MAXGTRIDSIZE

- removes a few unused initial values to silence NetBeans warnings

- removes unused Cursor#getUnicodeStream

Otherwise, the patch contains a few Javadoc improvements and typo
fixes.

git-svn-id: https://svn.apache.org/repos/asf/db/derby/code/trunk@1467726 13f79535-47bb-0310-9956-ffa450edef68