This change includes removing older azureblob storage SDK, and getting
parity to existing code with latest blob storage SDK.
This change is also pre-req for addressing #2091
* Fixup bitrot (rclone and Azure library)
* Implement Copy
* Add modtime to metadata under mtime key as RFC3339Nano
* Make multipart upload work
* Make it pass the integration tests
* Fix uploading of zero length blobs
* Rename to azureblob as it seems likely we will do azurefile
* Add docs