Rework use of storage clients

[Porges]

• Reuse Storage clients by caching them against account name.
  • This follows recommendations of Storage documentation: https://devblogs.microsoft.com/azure-sdk/lifetime-management-and-thread-safety-guarantees-of-azure-sdk-net-clients/
• Use strongly-typed ResourceIdentifier when passing around Account IDs, to prevent mixing them up with Account Names.
• Simplify & centralize SAS generation in one place.

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Abandoned changes:

• Use managed identity to access storage accounts instead of using shared key — no longer need to use management client to fetch shared keys.
  • This should improve overall performance since we don't need to fetch the keys.
  • As an exception to this, the queue client needs shared keys to generate SAS URIs.

This was abandoned since:

• we still need shared-key access for Queue SAS generation, so we can't remove shared-key access
• our current MSI token is not authorized for direct access to storage so we would need to add more roles to the deployment