Wrap EPM `Snapshot`, `SnapshotMetadata` and `DesiredTargets` to ensure invariants

[gpestana]

The Snapshot, SnapshotMetadata and DesiredTargets storage items in the EPM pallet must be kept in sync throughout the election lifetime and be killed in tandem. Currently, these storage items are mutated in different places in the pallet.

It makes sense to refactor EPM to hide those storage items behind a shared wrapper that is used to access/mutate and kill the inner items, so as to make sure the invariants hold true at every point in the election.

(related comment)

[wirednkod]

I would like to work on this @gpestana