Changes needed to implement the runtime part of elastic scaling:
https://github.com/paritytech/polkadot-sdk/issues/3131,
https://github.com/paritytech/polkadot-sdk/issues/3132,
https://github.com/paritytech/polkadot-sdk/issues/3202

Also fixes https://github.com/paritytech/polkadot-sdk/issues/3675

TODOs:

- [x] storage migration
- [x] optimise process_candidates from O(N^2)
- [x] drop backable candidates which form cycles
- [x] fix unit tests
- [x] add more unit tests
- [x] check the runtime APIs which use the pending availability storage.
We need to expose all of them, see
https://github.com/paritytech/polkadot-sdk/issues/3576
- [x] optimise the candidate selection. we're currently picking randomly
until we satisfy the weight limit. we need to be smart about not
breaking candidate chains while being fair to all paras -
https://github.com/paritytech/polkadot-sdk/pull/3573

Relies on the changes made in
https://github.com/paritytech/polkadot-sdk/pull/3233 in terms of the
inclusion policy and the candidate ordering

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Signed-off-by: alindima <[email protected]>
Co-authored-by: command-bot <>
Co-authored-by: eskimor <[email protected]>