HPCC-2284 Reduce size of the continuation info for Roxie indexes

No need to serialize the key buffer contents - can be retrieved from the index

Signed-off-by: Richard Chapman <rchapman@hpccsystems.com>
Merge pull request #4363 from richardkchapman/roxie-serialization

HPCC-2284 Reduce size of the continuation info for Roxie indexes

Reviewed-by: Gavin Halliday <ghalliday@hpccsystems.com>