S3A prefetching to support Vector IO

Add explicit support for vector IO in s3a prefetching stream.

• if a range is in 1+ cached block, it SHALL be read from cache and returned
• if a range is not in cache : TBD
• If a range is partially in cache: TBD

these are the same decisions that abfs has to make: should the client fetch/cache block or just do one or more GET requests

A big issue is: does caching of data fetched in a range request make any sense at all? Or more specifically: does fetching the blocks in which range requests are found make sense

Simply going to the store is a lot simpler