Ensure referenda `TracksInfo` is sorted (#3325)

Changes:
- Add `integrity_check` to trait `TracksInfo` to assert the sorting 
- Use the check in `integrity_test`
- Rely upon sorted property in the `info` for speedup

Note that the pallet already relies upon this (so far) untested
assumption
[here](https://github.com/paritytech/polkadot-sdk/blob/44c7a5eb8c375d77f685abf585961421e5f8b228/substrate/frame/referenda/src/lib.rs#L1280

).

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Signed-off-by: Oliver Tale-Yazdi <[email protected]>
Co-authored-by: Bastian Köcher <[email protected]>