Overlapping shards with duplicate points create query problems

[lesam]

Steps to reproduce:
List the minimal actions needed to reproduce the behavior.

1. Create overlapping shards (by changing the retention policy duration)
2. Write points into the new shard that match points in the old shard
3. select sum(overlapping_field) from ...

Expected behavior:
Never allow overlapping shards as a result of retention policy duration

Actual behavior:
Matching points are summed separately by the query engine