Tweak Rewards API: Unique ID + do not sum rewards per layer

[brusherru]

Rationale

There are two issues with GlobalStateService.AccountDataQuery and GlobalStateService.AccountDataStream regarding Rewards.

The first one is that the output of these endpoints is a bit inconsistent:

• AccountDataQuery returns the sum of rewards per layer. So it returns only one Reward object per layer.
• AccountDataStream may return many rewards per layer.

The second one is that we don't have any unique ID in the Reward object, which makes it impossible to get rid of duplicates.
We need to add SmesherID / BallotID / AtxID or any other unique ID.

Required by

• Explorer: to get rid of duplicates
• Smapp: to get rid of duplicates & keep storing data on fs :)

Additional info

We already have an issue to add SmesherID into the Reward object: spacemeshos/go-spacemesh#4529