Currently the `para_backing_state` API is used only by the prospective
parachains subsystems and returns 2 things: the constraints for
parachain blocks and the candidates pending availability.

This PR deprecates `para_backing_state` and introduces a new
`backing_constraints` API that can be used together with
`candidates_pending_availability` to get the same information provided
by `para_backing_state`.

TODO:
- [x] PRDoc

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Signed-off-by: Andrei Sandu <andrei-mihail@parity.io>
Co-authored-by: command-bot <>