Update reduceCapacity to support new data shape

[AlexanderBelokon]

When using batch operations the stats are not returned correctly.

For example, this request:

batchWriteAll({
  RequestItems: { ... },
  ReturnConsumedCapacity: 'INDEXES',
  ReturnItemCollectionMetrics: 'SIZE'
})

causes AWS to return the following data structure:

{
  ConsumedCapacity: [{
    TableName: 'table',
    CapacityUnits: 8,
    Table: { CapacityUnits: 4 },
    GlobalSecondaryIndexes: { index: { CapacityUnits: 4 } }
  }]
}

which is then transformed by reduceCapacity inside sendAll to this:

{
  ConsumedCapacity: {
    TableName: undefined
  }
}

This PR fixes that problem.

[AlexanderBelokon]

Noticed that there were a few more fields missing from the current doc:

Items: Array<Map<Map>>
    Count: Integer
    ScannedCount: Integer
    LastEvaluatedKey: Map<Map>
    ConsumedCapacity: Map
        TableName: String
        CapacityUnits: Float
        ReadCapacityUnits: Float
        WriteCapacityUnits: Float
        Table: map
            ReadCapacityUnits: Float
            WriteCapacityUnits: Float
            CapacityUnits: Float
        LocalSecondaryIndexes: Map<Map>
            ReadCapacityUnits: Float
            WriteCapacityUnits: Float
            CapacityUnits: Float
        GlobalSecondaryIndexes: Map<Map>
            ReadCapacityUnits: Float
            WriteCapacityUnits: Float
            CapacityUnits: Float

[davidweimapbox]

should we check if src or dst is null in case?

[AlexanderBelokon]

src can't be null here because we're not calling this function if it is (on L55) (TypeScript would be helpful in enforcing that, but we don't have it here), and we init dst on L46-47.

There's a test case already that checks this code path: [reduceCapacity] parses new data format correctly, there dst is built up from nothing.

But you're right that this is not obvious at all without compiler's help and someone in the future might break that accidentally. The tests would show it, though.

[davidweimapbox]

Normally, I would like to add null/undefined checking before accessing its properties to avoid unexpected exception thrown, especially this is an utils function, try to make it more common and robust.

[davidweimapbox]

LGTM.