Explicitly release operation states early to release tiles for continuations

[msimberg]

If we end up doing 1. or 2. from pika-org/pika#479 we need to make changes in DLA-Future. Currently we rely on the pika implementation to release operation states/tiles early. The P2300 reference implementation does not release operation states early. This issue is meant to track whatever change we end up needing to do. I will most likely first go with an explicit sender adaptor that allows releasing the operation state early in tactical places.

[msimberg]

Will be fixed by #1004.