After the multipart chunker refactor the multipart chunked server side
copy was accidentally sending one part too many. The last part was 0
length which was rejected by b2.
This was caused by a simple off by one error in the refactoring where
we went from 1 based part number counting to 0 based part number
counting.
Fixing this revealed that the metadata wasn't being re-read for the
copied object either.
This fixes both of those issues and adds an integration tests so it
won't happen again.
See: https://forum.rclone.org/t/large-server-side-copy-in-b2-fails-due-to-bad-byte-range/42294