DERBY-6125 Code cleanup in client driver

Patch derby-6125-visibility-01-b; details

- limits visibility of many classes, methods and fields to what's
  necessary,

- adds final specification to ClobWriter#clob_ and

- removes ClientPooledConnection.getNetXAConnection by inlining its
  content in the one usage location.

git-svn-id: https://svn.apache.org/repos/asf/db/derby/code/trunk@1465956 13f79535-47bb-0310-9956-ffa450edef68