HPCC-12010 Fix deadlock in onActivity cache building

If building, whilst a queue request is processing, e.g.
onSetJobPriority, the two threads involved can deadlock each
other.
Regression introduced by HPCC-11637

Signed-off-by: Jake Smith <jake.smith@lexisnexis.com>
HPCC-12010 Additional changes following review

Signed-off-by: Jake Smith <jake.smith@lexisnexis.com>
Merge pull request #6298 from jakesmith/hpcc-12010

HPCC-12010 Fix deadlock in onActivity cache building

Reviewed-by: Gavin Halliday <ghalliday@hpccsystems.com>