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>
Community Edition 4.2.6-3 Gold

Signed-off-by: Gavin Halliday <gavin.halliday@lexisnexis.com>
Merge remote-tracking branch 'origin/candidate-4.2.6' into candidate-4.2.8

Signed-off-by: Gavin Halliday <gavin.halliday@lexisnexis.com>

Conflicts:
	version.cmake
Community Edition 4.2.8-rc4 Release Candidate 4

Signed-off-by: Gavin Halliday <gavin.halliday@lexisnexis.com>