Scoru: support EOL type of input in the inbox
An inbox will (for now) contain:
- Regular inputs
- EOL (end-of-level) input
Each Tezos level will have a EOL, that is, there would not be anymore empty inboxes.
It's useful both to
- Pave the way for DAL (new type of inputs)
- Allow a tick evaluation for each level, even if there are no messages