HPCC-10367 Rename priority to cost - to clarify higher costs done later

Signed-off-by: Gavin Halliday <gavin.halliday@lexisnexis.com>
Merge pull request #5179 from ghalliday/issue10367

HPCC-10367 Rename priority to cost - to clarify higher costs done later

Reviewed-By: Jake Smith <jake.smith@lexisnexis.com>
Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>