INT-3216 MGS Reaper Improvements

JIRA: https://jira.springsource.org/browse/INT-3216

Previously, After a group has been selected for completion
by the reaper, if the group is otherwise completed before
the reap AND expireGroupsOnCompletion is true AND a new
group with the same ID is created, ALL in the same millisecond,
AND the group creation timestamp is used for the reap criteria,
the new group will be incorrectly reaped.

Also, for a simple MGS, it was not necessary to re-fetch the group
if the group has already been marked complete.

- Defer reaping if the group is already complete (unless it's empty).
- Verify the timestamp hasn't changed (indicating a new group)

Add tests

- Verify no refetch of a complete SMG
- Verify no reap of a complete group (after refetch)
- Verify no reap when the timestamp changed