HPCC-9366 diskaggregate leaking threads

Signed-off-by: Jake Smith <jake.smith@lexisnexis.com>
Merge pull request #4399 from jakesmith/diskagg-leak

HPCC-9366 diskaggregate leaking threads

Reviewed-By: Gavin Halliday <gavin.halliday@lexisnexis.com>
Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>