[Orchestrator] Use server side copy for backups

[loic-sharma]

The Orchestrator backs up a package at the beginning of the validation (see this code). This is done by downloading the package from Blob storage to disk, and then uploading from disk to blob storage. We could instead do a Blob storage server-side copy. We would need to get the blob's hash somehow without downloading the blob.

This improvement is necessary as Orchestrator messages that take a long time to process cause message duplication and deadlettering. Here is a validation set that performed a backup that took 2 minutes:

traces
  | where cloud_RoleName == "NuGet.Services.Validation.Orchestrator" 
  | extend CallGuid = tostring(customDimensions.CallGuid)
  | where CallGuid == "8e662cec-d9ad-4955-8d28-a5b2d7632cc4"
  | order by timestamp desc
  | where timestamp >= todatetime('2018-11-20T00:36:40.459')
  | where timestamp <= todatetime('2018-11-20T00:38:33.175')