Refactors staking and `stake-tracker` so that target and voter lists include idle stakers (#2497)

In this PR we change the current assumptions so that `TargetList` and
`VoterList` **also** keep track of the idle stakers (w.g. chilled) and
the target scores are tracked and updated even while the validator is
chilled. The idle stakers will be removed when the ledger unbonds.

This allows the stake-tracker to keep track of the chilled validators
and respective score so that a re-validate of a chilled validator is
brought up with the correct target score (i.e. self stake + all current
nominations in the system).

---------

Co-authored-by: command-bot <>