[DynamoDB] Use variable rate limiting for DynamoDB workloads where the item size varies

[amcp]

Each KB rounded up of data written to DynamoDB consumes one unit of WCU from provisioned throughput. Rate limiting writes on a per-request basis without taking into account the size of the item being written (current state in ndbench) will cause unexpected server-side throttling when item sizes vary. This issue exists to implement variable rate limiting for reads and writes. A reference implementation can be found in the DynamoDBDelegate.java class in the DynamoDB Storage Backend for JanusGraph.

• variable rate limiting for writes is deterministic
• variable rate limiting for reads is not deterministic and requires metering the consumed capacity of each read request to estimate.

[amcp]

depends on #153