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Write a component that handles uploading data for a given contact to a given mailbox. This upload queue component will be managed by the upload queue manager (#2228).

When the queue is created:

• Check that the mailbox is reachable (#2170 for own mailbox, #2186 for a contact's mailbox)
• Look up the retry info for any failed uploads to this contact (#2230)
• For each failed upload:
  • If interrupted, return
  • If the file doesn't exist or is empty:
    • Remove the retry info from the DB
  • Else:
    • Upload the file (#2231)
    • If the upload succeeds:
      • Delete the file
      • Remove the retry info from the DB
• Until interrupted:
  • If there are any acks or messages to send:
    • Create a temporary filename
    • Store the retry info (contact ID and filename) in the DB
    • Call the mailbox plugin to write a stream to the file
    • Upload the file
    • If the upload succeeds:
      • Delete the file
      • Remove the retry info from the DB
  • Else:
    • Wait for the next send time, a notification from the manager, or an interrupt from the manager

The manager will notify the queue of events that may cause acks or messages to become sendable to the contact (MessageToAckEvent, MessageSharedEvent, GroupVisibilityUpdatedEvent).

Depends on #2170, #2186, #2230, #2231.