DERBY-6737: CLOB retrieve exceptions after moving cursor around

Always fetch the row again from the server when moving the position of
a scrollable result set that contains LOB columns.

Without this fix, if one of the absolute positioning methods is used
(first(), last() or absolute(int)), and the old position is the same
as the new position, the result set will use the values it already has
for the row on that position. Any locators will have been released,
though, so accessing LOBs in the row will fail with 'invalid locator'.

By fetching the row again from the server, we get a fresh and valid
locator for the LOB columns in that row.

git-svn-id: https://svn.apache.org/repos/asf/db/derby/code/trunk@1625904 13f79535-47bb-0310-9956-ffa450edef68