HPCC-10073 Allow a spill priority to be specified on an allocation

Signed-off-by: Gavin Halliday <gavin.halliday@lexisnexis.com>
Merge pull request #5072 from ghalliday/issue10073

HPCC-10073 Allow a spill priority to be specified on an allocation

Reviewed-By: Jake Smith <jake.smith@lexisnexis.com>
Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>