S3Path: support extremely fast bucket-to-bucket transfers

[pjbull]

We've experimented with this tool for fast bucket-to-bucket transfers. It would be cool if we could leverage the logic here to make an incredibly fast bucket-to-bucket transfer:
https://github.com/generalui/s3p/

Here's a blog post with more deets:
https://medium.com/@shanebdavis/s3p-massively-parallel-s3-copying-9a9e466d0d74

I think the core pieces are:

• async for all of the requests
• use prefix and assumptions about list results in alphabetical order to split up the parallel "list" calls search space? (or something like that)
• get a massive number of requests goin' at once.

(This may also be out of scope for the package, just throwing it out there as useful for our purposes)