Transfer: reset local_path for transfers where the file is no longer shared

[JurgenR]

When an item is no longer shared but was previously requested the local_path of a transfer might still be set. This might cause the file still to be uploaded despite no longer wanting to share it.

If the shared item is removed from the shares:

• Complete / Failed: should do nothing -> ensure local_path is cleared when item goes back in queue (although this should not be possible)
• Uploading: not sure yet? Finish it or cancel?
• Queued: remove the local_path, move to Failed?

[JurgenR]

I feel like the logic for state transitions could still be simplified, it seems like such odd situations keep occurring

[JurgenR]

As part of this issue, also check what happens if the file has become locked for that user

[JurgenR]

What might resolve issues is to simply check if the file is accessible when a PeerTransferQueue or PeerTransferRequest comes in and only set the local_path when the download or upload starts.