Merge pull request #4227 from jakesmith/hpcc-9127

HPCC-9127 - Fix global merge regression

Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>
HPCC-9120 - Fix global rollup early stop deadlock

If global rollup was stopped before it had process all it's input,
it would stall on a multi slave setup. If stopped, global rollup
was not signalling to the next nodes that it had finished.

Signed-off-by: Jake Smith <jake.smith@lexisnexis.com>