Add support for multiple snapshot intervals to SnapshotPolicy

At the moment, the `SnapshotPolicy` has a single snapshot interval which determines the period between slots for which snapshots should be taken. To support Mithril, we need to be able to take both predictable snapshots and the existing periodic snapshots within the same node.

In order to support this, we need to allow node operators to configure their nodes with multiple snapshot intervals and ensure that the intervals are treated independently within the snapshotting mechanism.