S3A copy/rename of large files to be parallelized as a multipart operation

AWS docs on copying

• file < 5GB, can do this as a single operation
• file > 5GB you MUST use multipart API.

But even for files < 5GB, that's a really slow operation. And if HADOOP-16188 is to be believed, there's not enough retrying.
Even if the transfer manager does swtich to multipart copies at some size, just as we do our writes in 32-64 MB blocks, we can do the same for file copy. Something like

l = len(src)
if L < fs.s3a.block.size: 
   single copy
else: 
  split file by blocks, initiate the upload, then execute each block copy as an operation in the S3A thread pool; once all done: complete the operation.

+ do retries on individual blocks copied, so a failure of one to copy doesn't force retry of the whole upload.

This is potentially more complex than it sounds, as

• there's the need to track the ongoing copy operational state
• handle failures (abort, etc)
• use the if-modified/version headers to fail fast if the source file changes partway through copy
• if the len(file)/fs.s3a.block.size > max-block-count, use a bigger block size
• Maybe need to fall back to the classic operation

Overall, what sounds simple could get complex fast, or at least a bigger piece of code. Needs to have some PoC of speedup before attempting