Bugfix: Allow uploading large files to MS Azure

Sometimes, restic creates files to be uploaded to the repository which are
quite large, e.g. when saving directories with many entries or very large
files. The MS Azure API does not allow uploading files larger that 256MiB
directly, rather restic needs to upload them in blocks of 100MiB. This is now
implemented.

https://github.com/restic/restic/issues/1822
