Change pallet referenda TracksInfo::tracks to return an iterator (#2072)

Returning an iterator in `TracksInfo::tracks()` instead of a static
slice allows for more flexible implementations of `TracksInfo` that can
use the chain storage without compromising a lot on the
performance/memory penalty if we were to return an owned `Vec` instead.

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Co-authored-by: Pablo Andrés Dorado Suárez <hola@pablodorado.com>