Call to listObjectsV2 should be modified to handle +1K objects

[rdmurphy]

This is currently an intentional limitation of the library — the call to aws-sdk-js's s3.listObjectsV2 function only returns 1,000 objects, and expects you to further paginate to get more than this.

For the majority of projects this probably isn't an issue. Until it is! (FWIW we've never hit it at the LA Times, I think we hit it once or twice at Texas Tribune over the span of like 5-6 years.)

Now that it's acceptable to have our minimum supported Node version as v10, we can use async iterators to make this a cinch.