With Coretime enabled we can no longer assume there is a static 1:1
mapping between core index and para id. This mapping should be obtained
from the scheduler/claimqueue on block by block basis.

This PR modifies `para_id()` (from `CoreState`) to return the scheduled
`ParaId` for occupied cores and removes its usages in the code.

Closes https://github.com/paritytech/polkadot-sdk/issues/3948

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Co-authored-by: Andrei Sandu <[email protected]>