Since 2.1, the aggregator maintains message group state in order to discard late-arriving messages.
However, forceComplete skips empty groups. This means, even if a MessageGroupStoreReaper is configured to expire groups, such groups are never expired (removed from the GroupMessageStore).
Empty groups should be considered eligible for reaping.
In 3.0, we might want to consider supporting 2 reaper cycles - reaping incomplete groups Vs. reaping empty groups.