DAL/Node: backfill statuses at startup

What

Backfill status info at startup with info from the last attestation_lag levels.

There's also a commit in order to not have a lot of useless warnings like this: WARN │ Slot header status not found for level 1, slot index 0.

Why

Because this information may be useful, see for instance !19948 (comment 2899260751).

Manually testing the MR

Added a simple test Alpha: Testing DAL node (Status information is backfilled at restart).

On master that test fails with

09:59:45.410] [RPC] GET http://127.0.0.1:47073/levels/2/slots/0/status
[09:59:45.411] [RPC] RPC response: 404 Not Found