[DAL]: Rollup node logic for downloading messages

Desired behaviour:

When processing a head, the rollup node checks to which slots it is subscribed and

1. If no slot subscriptions are found, it will look at the operations in L1 to download the list of messages (current behaviour)
2. If at least one slot subscription is found, the messages will be downloaded from the subscribed slots after lag levels have passed and the slot is marked as available.