Support multipart-put benchmarking

[d1tyo]

Proposed solution (warp multipart-put) is aimed to benchmark uploading of multiple multipart objects concurrently. It periodically creates new multipart uploads and concurrently uploads parts.

[harshavardhana]

You don't like warp multipart ?

[d1tyo]

You don't like warp multipart ?

warp multipart doesn't allow to benchmark multipart upload itself. It mostly benchmarks download and only uploads to a single multipart upload id.

Proposed solution is aimed to benchmark uploading of multiple multipart objects concurrently. It periodically creates new multipart uploads and concurrently uploads parts.

So proposed warp multipart-upload benchmarks a different scenario from warp multipart.

[klauspost]

Sorry about the name change. Rest should be trivial.

[d1tyo]

Just updated PR. I renamed both multipart_upload.go files to multipart_put.go as well.
Please take a look.

[d1tyo]

Updated it once again.

[klauspost]

ltgm 👍🏼