HPCC-8938 - Exception can cause sort partition deadlock

Need to ensure next in asyncfor loop is signalled to continue
to avoid deadlock.
Also add a stack trace when roxiemem runs out of memory.
Merge pull request #4430 from jakesmith/hpcc-8938

HPCC-8938 - Exception can cause sort partition deadlock

Reviewed-By: Gavin Halliday <gavin.halliday@lexisnexis.com>
Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>