Create regular snapshots for resources with a high number of revisions

**Motivation**
Some resources have a lot of revisions and replaying the events to get to a particular revision could be sped up by making regular snapshots.

**Acceptance criteria**
1. Each event log would include a routine that asynchronously computes snapshots every n revision of a resource
2. This routine is available for both global and scoped entities
3. The n should be configurable
4. The snapshots are used to compute the state of the revision at revision r