Allow joining over `FlaggedStorage` in parallel

## Problem

We cannot join over a `FlaggedStorage` as usual because it does not fulfill the `DistinctStorage` guarantee (since it pushes events when accessed).

## Potential solution

We can push the events prior or after the actual access and keep it out of any parallel logic. This seems to be the technically most reasonable solution, but might be very hard to get right, especially with the API we have.