JCLOUDS-457: BlobStore MultiPartUpload strategy

The code related to the MultiPartUpload strategy has been added.
MultiPart uploads use an upload strategy (e.g. sequential vs parallel)
and also a slicing strategy to split the payload in different parts.
Avoid duplication with:

```
long sliceLength = Math.min(getRemaining(), partSizeInMB << 20);
```