DAL/Node: update slot statuses correctly at migration

What

The main goal of this MR is to correctly update statuses at migration in case of a change of attestation_lag.

The MR also tests the new status RPC.

The MR also has a few additional small improvements (see commits, many are trivial).

I also took two commits updating the same skip-list RPC test from !19442 (merged), because I tested it on top of it, so it was easier to have these commits.

Closes #8079 (closed).

Testing

dune exec tezt/tests/main.exec -- -f dal.ml alpha skip_list

And one can do the same on top of !19442 (merged).