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/44c7a5eb

/substrate/frame/referenda/src/lib.rs#L1280).

---------

Signed-off-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>
Co-authored-by: Bastian Köcher <git@kchr.de>