HPCC-10660 : Improve efficiency of subscription session remove

Convert linear lookup to hash table lookup

Signed-off-by: Jake Smith <jake.smith@lexisnexis.com>
Merge pull request #5282 from jakesmith/hpcc-10660

HPCC-10660 : Improve efficiency of subscription session remove

Reviewed-By: Gavin Halliday <gavin.halliday@lexisnexis.com>
Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>
Community Edition 4.2.2-rc7 Release Candidate 7

Signed-off-by: Richard Chapman <rchapman@hpccsystems.com>
Merge remote-tracking branch 'origin/candidate-4.2.2' into closedown-4.2.x

Conflicts:
	version.cmake

Signed-off-by: Richard Chapman <rchapman@hpccsystems.com>