Mailbox client superclass

Mailbox clients for communicating with our own mailbox and contacts' mailboxes are managed by a singleton mailbox client manager (#2228 (closed)).

We'll need different client implementations for our own mailbox (#2290 (closed)) and contacts' mailboxes (#2289 (closed)), but there will be a shared interface and some shared code. This ticket covers the code shared between the client for our own mailbox and the client for a contact's mailbox.

Interface:

• The manager can create and destroy clients
• The manager can assign/deassign contacts for upload or download
• The client's upload and download workers can request connectivity checks

Implementation:

• At any given time the client may have multiple upload workers and/or a single download worker
• The client has a connectivity check method, which takes an observer as an argument
• The client stores the time of the latest successful connectivity check, a reference to the current connectivity check task, if any, and a list of observers waiting for the result of a connectivity check

When the client is destroyed:

• If a connectivity check task is running:
  • Cancel the connectivity check task
• Destroy the upload workers, if any
• Destroy the download worker, if any

When the connectivity check method is called:

• Compare the current time to the time of the latest successful connectivity check
• If a connectivity check has recently succeeded:
  • Notify the observer
• Else if a connectivity check task is running:
  • Add the observer to the list of waiting observers
• Else:
  • Add the observer to the list of waiting observers
  • Start a connectivity check task

When a connectivity check task succeeds:

• Update the time of the latest successful connectivity check
• Notify any waiting observers
• Clear the list of waiting observers