when multiple strategies for the same volume, don't delete unless no other strategy is pointing to that snapshot

[milesmatthias]

hours_between: 1
num_to_keep: 2
mount_point: /dev/sda1
pre_snapshot_tasks: 
post_snapshot_tasks: 
|snap-5bd90897:vol-9c52bcb4,snap-a2f9286e:vol-9c52bcb4|active|1|1|2|||/dev/sda1

---
hours_between: 24
num_to_keep: 4
mount_point: /dev/sda1
|snap-6479aca8:vol-9c52bcb4|active|3|24|4|||/dev/sda1

---
hours_between: 96
num_to_keep: 4
mount_point: /dev/sda1
|snap-6479aca8:vol-9c52bcb4|active|75|96|4|||/dev/sda1

snap-6479aca8 doesn't exist anymore because the first strategy deleted it, since it didn't check to see if any other strategies have a pointer to that snapshot before deleting it in EC2.