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.