JCLOUDS-356 multipart uploads with InputStream payloads


This changeset introduces an alternative to `PayloadSlicer`, called `IterablePayloadSlicer`, and updates `org.jclouds.openstack.swift.blobstore.strategy.internal.SequentialMultipartUploadStrategy` to make use of it.

https://issues.apache.org/jira/browse/JCLOUDS-356