INT-2937 Fix Race Condition in MGS Reaper

Groups changed within the same millisecond that they
were selected for reaping can cause aggregators to
emit duplicate messages.

Use the re-fetched group (used to see if the timetamp
changed) instead of the group passed in to forceClose
(the group might have changed since it was selected as
eligible for reaping).

We still retain the last modified check because it
might have been expired using timeoutOnIdle.
Merge pull request #751 from garyrussell/INT-2937

* INT-2937: Fix Race Condition in MGS Reaper