While investigating some pruning issues I found some room for
improvement in the notification pin handling.

**Problem:** It was not possible to define an upper limit on
notification pins. The block pinning cache has a limit, but only handles
bodies and justifications.

After this PR, bookkeeping for notifications is managed in the pinning
worker. A limit can be defined in the worker. If that limit is crossed,
blocks that were pinned for that notification are unpinned, which now
affects the state as well as bodies and justifications. The pinned
blocks cache still has a limit, but should never be hit.

closes #19

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Co-authored-by: Bastian Köcher <[email protected]>
Co-authored-by: André Silva <[email protected]>