DAL/Baker: retrieve DAL attestations in advance

(Follow-up from "Baker: introduce dal-node-timeout-percentage argument")

Given that the attestation_lag is quite big, we can fetch the status "in advance", that is, at the start of the round or even at the start of the level. (Only do it if the baker has TB slots at the current level, or even only if it has DAL shards at the current level).

This would also allow to wait for a fixed amount of time (corresponding to the expected RPC latency) instead of a un-intuitive percentage.