Block and unblock a vnode when triggering a repair.  Otherwise their is a potential race condition:

- aae_controller is triggered into repair mode, queueing all updates to be re-applied after rebuild complete
- update is applied to leveled backend by vnode
- snapshot is taken for rebuild (including update)
- message is cast to aae_controller with update ... which will be queued and applied twice
- update is queue

If the selective receive times out - then need to handle the release message