DAL/Slot manager: don't reconstruct if slot is already stored

What

Stacked on !12875 (merged).

Lookup the slot store before attempting a reconstruction in the GET /slot/pages/ RPC.

Why

Performance.

How

The function responsible for computing the slot is Slot_manager_legacy.get_slot. In the first commit, we grant it access to the full store instead of only the shard part. In the second commit, we use this to try to read the slot from the slot store and only perform a reconstruction from the shards if the slot could not be obtained.

Manually testing the MR

Checklist

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