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Create a manager interface in bramble-api, with an impl in bramble-core, to manage access to the RemovableDrivePlugin (#2015 (closed)). The manager will provide methods for doing the following:

• Retrieve the current reader/writer task, if any, for a given contact
• Register and start a reader/writer task for a contact (fails if an equivalent task is already registered)
• Unregister a task (for use by the task itself, when it finishes running)

We may later want to convert this to some kind of abstract TaskManager interface with TaskKeys and so on, and refactor other long-running tasks to use it, but let's not do that yet.

Subtask of #1802