HPCC-9860 - Avoid holding session crit whilst destroying subs

A destructing subscription can involve other locks/crits.
The session manager should ensure the session crit is released
before their final destruction. Otherwise deadlock can ensure
if another thread is involved in e.g. locking and queries the
session manager.

Signed-off-by: Jake Smith <jake.smith@lexisnexis.com>
Merge pull request #4744 from jakesmith/hpcc-9860

HPCC-9860 - Avoid holding session crit whilst destroying subs

Reviewed-By: Gavin Halliday <gavin.halliday@lexisnexis.com>
Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>