PoS - use finer-grained storage keys

[tzemanovic]

Depends on #12.

Refactor the PoS integration's storage keys to store the epoched data split up into finer-grained data. Currently, all the epoched data are serialized as the whole Epoched/EpochedDelta value.

For example, a validator's voting power is stored in "{PoS_address}/validator/{validator_address}/voting_power". Instead, we can store the fields of the Epoched/EpochedDelta split up into their fields:

• "{PoS_address}/validator/{validator_address}/voting_power/last_update"
• "{PoS_address}/validator/{validator_address}/voting_power/{n}" for each set value in the inner array

This way, when a transaction changes this data, we don't have to find the diff in the whole structure for validation and the validation should then be simpler.

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[tzemanovic]

This is even more important for the validator sets field - currently, the only way to validate its changes in the VP is to iterate the validator sets to check for expected and unexpected changes.

For validator sets, it would be useful to have a "LazyMap/Set" built on top of the tx/VP interface, in which the map/set's keys would be formatted into storage sub-keys, so that VP would only receive the entries that have actually changed, without having to look for diffs in the whole collection. This depends on #168

[cwgoes]

@brentstone do you know what the status of this is?

[tzemanovic]

I think this will be closed by #740

[brentstone]

Closed by #740.