1. Fix handling of PUT. The simple implementation was the correct
implementation, I have no idea what was going on here.
2. Use MFS everywhere to reduce code duplication and add support for sharded
directories.
3. _Correctly_ block IPNS.
4. Remove the dependency on `core.IpfsNode`.
5. Remove support for putting empty directories with a well-known CID. It was
useless as directories are automatically created.
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