186-Store-PoolSnapshot-data-on-epoch-close-linked-to-Epoch

[filo87]

closes #186
closes #221

BREAKING CHANGES:
Aggregated NAV queries in UI should now be constructed like this:

{
  snapshotPeriods(filter: { weekDay: { equalTo: 1 } }, orderBy: START_DESC, first: 1000) {
    nodes {
      id
      poolSnapshots {
        aggregates {
          sum {
            normalizedNAV
          }
        }
      }
    }
  }
}

Filed periodStart is DEPRECATED on all snapshots. Substituted with a foreigh key periodId to a SnapshotPeriod entity.