feat(batch): optimize selection of registration for risk evaluation

Instead for selecting all registrations with non-empty exposed periods, we keep track in the registration of the need to recompute the risk (see outdatedRisk boolean field). This restricts the selection to necessary entries, and makes it possible to use a mongo index.

The mongo index should be added as follows:

db.idTable.createIndex( { outdatedRisk: 1 }, { partialFilterExpression : { outdatedRisk: true } } )