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>