Reduce buffered data for multipart upload

[artttt]

Currently the distributed multipart upload keeps whole input bytes objects as the "first part's" buffer.
An additional step could be added where 5000 rather then 10000 partitions are created but then the first 5MiB of each partition is split off into its own partition.
This means only 5MiB is kept in memory while waiting for more data to continue the reduction.