Adds target approvals buffering below a threshold

[gpestana]

This PR adds target stake updates buffering to stake-tracker. It exposes a configuration that defines a threshold below which the target score should not be updated automatically in the target list. The Config::ScoreStrictUpdateThreshold defines said threshold. If a target stake update is below the threshold, the stake update is buffered in the UnsettledTargetScore storage map while the target list is not affected. Multiple approvals updates can be buffered for the same target. Calling Call::settle will setttle the buffered approvals tally for a given target. Setting Config::ScoreStrictUpdateThreshold to None disables the stake approvals buffering.

• benchmarks
• try-state checks considering the unsettled score
• docs

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[kianenigma]

Sorry for being late, should have reviewed this sooner.

I think we should revert and re-do this, as it is different from what I believe was the intended fix.

The original issue is that each ledger update in a nominator can correspond to up 16 approval stake updates. With this PR, we have reduced that to 1, but I think we can do better.

I belive what we actually need to store is not Unsettled but instead LastSettled. This is a mapping from each nominator to their last tracked approval stake, and keeps track of the last time that a nominator's stake was counted towards validators approval stake.

As rewards and slashes come in, we do nothing. So suppose LastSettled(alice) = 100, meaning that upon our last calculation, Alice had 100 stake, and it had contributed 100 to a set of up to 16 validators.

Then, once alice gets reward and slashes, and now she has 200, anyone can call re_balance(alice). This call will update LastSettled(alice) to 200, and also update the approval stake of all validators that alice has backed.

This approach is a lot less computation and weight update, and a lot more storage, which is totally fine in a parachain.