Batch by total request message size.

[petermyers]

Per the aws docs for SendMessageBatch:
The maximum allowed individual message size and the maximum total payload size (the sum of the individual lengths of all of the batched messages) are both 256 KiB (262,144 bytes).

• Adds a batchWithPayloadSizeOrCount that is JUST batching and doesn't deal with any of the claimcheck logic.
• Adds batching per request size in addition to the message count to the SQS sink.